Question 1

In a TCP/IP network, what is the primary purpose of a DHCP (Dynamic Host Configuration Protocol) server?

Accepted Answer

Introduction / Context: Modern networks often contain many end devices, and manually configuring every host with an IP address, subnet mask, default gateway, and DNS information is time-consuming and error prone. DHCP was designed to automate this process and centralize IP address management, making network administration more efficient and reducing configuration mistakes. Given Data / Assumptions: We are working in an IPv4 environment using TCP/IP. Hosts require IP addresses and related settings to communicate on the network. A server running the DHCP service is available on the network. The question asks specifically about the main purpose of this server. Concept / Approach: DHCP is responsible for dynamically allocating IP configuration to clients. When a client boots and requests an address, the DHCP server selects an available address from its configured scope and sends it to the client, along with the subnet mask, default gateway, DNS server addresses, and possibly other options. This eliminates the need for static configuration on each device and allows addresses to be reused efficiently through leases. The correct answer must focus on automatic IP configuration, not on unrelated services like email or name resolution. Step-by-Step Solution: 1. Identify the core need: hosts must obtain IP configuration before they can communicate.2. Recognize that DHCP is specifically designed to assign IP addresses and related parameters dynamically.3. Note that tasks such as translating URLs to IPs belong to DNS, not DHCP.4. Translating IP to MAC on the local segment is performed by ARP, not by DHCP.5. Therefore, the best description of DHCP's purpose is automatic provision of IP configuration information to hosts. Verification / Alternative check: You can verify this behavior on a client by observing the DHCP negotiation process using packet capture tools. The client sends a DHCPDISCOVER, the server replies with an offer, and then the client requests and receives an IP configuration. This exchange clearly shows that the DHCP server distributes IP configuration parameters, not email, DNS name resolution, or MAC translations. Why Other Options Are Wrong: Option A describes an email or messaging server, which is unrelated to DHCP. Option B describes a Domain Name System server, whose job is to map domain names to IP addresses. Option C describes the Address Resolution Protocol, which resolves local IP addresses to hardware addresses. None of these accurately state the primary purpose of DHCP. Common Pitfalls: Learners sometimes confuse DHCP with DNS because both are involved in network configuration. Others confuse it with ARP because they think of address resolution in general terms. Remember that DHCP handles the initial configuration of IP parameters, while DNS and ARP solve different but complementary problems on the network. Final Answer: To automatically provide IP configuration information, such as IP address, subnet mask, and default gateway, to hosts.

Question 2

An administrator on host PC1 issues the command ping 127.0.0.1. Which conclusion can be drawn if this ping succeeds?

Accepted Answer

Introduction / Context: The ping utility is a basic but powerful tool for diagnosing network connectivity problems. When troubleshooting a host, one of the first tests is often a ping to the loopback address 127.0.0.1. Understanding what a successful or unsuccessful response means helps you quickly determine whether the issue lies on the local machine or elsewhere in the network path. Given Data / Assumptions: The administrator is working at host PC1. The command ping 127.0.0.1 is executed. The ping to the loopback address succeeds, returning replies. No additional tests to other hosts have yet been attempted. Concept / Approach: The IPv4 address 127.0.0.1 is reserved as the loopback address and always refers to the local host. Traffic to this address never leaves the host; it is processed by the local TCP/IP stack. A successful ping to 127.0.0.1 proves that the IP protocol stack is correctly installed and functioning on the machine. However, it does not verify connectivity to the LAN, default gateway, or any external device, because the test does not traverse the network interface card or any cables or switches. Step-by-Step Solution: 1. Recognize that 127.0.0.1 is the standard loopback address defined for IPv4.2. When pinging 127.0.0.1, the operating system sends ICMP echo requests directly to its own network stack.3. The packet does not exit the host onto the physical network medium.4. A successful reply means that the local TCP/IP implementation is working and can process ICMP messages.5. Therefore, the only safe conclusion is that the TCP/IP protocol stack is functioning correctly on PC1. Verification / Alternative check: If you disable the TCP/IP protocol or corrupt its configuration, pinging 127.0.0.1 will typically fail, showing that the loopback test depends on a functional local stack. You can then progress to additional tests such as pinging the host's own IP address, the default gateway, and remote hosts to further isolate network problems. Why Other Options Are Wrong: Option A is incorrect because the test does not involve any other host. Option B is incorrect because it does not confirm connectivity to a router or any Layer 3 device. Option C is incorrect because the default gateway configuration is not used when pinging 127.0.0.1. Option D is misleading because ping only verifies up to the network layer and ICMP; it does not confirm transport or session layer connectivity to external devices. Common Pitfalls: Many learners overinterpret a successful loopback ping, assuming that the physical network is also healthy. In reality, you must test external addresses to verify cabling, switch ports, and router connectivity. Another pitfall is ignoring a failed loopback ping, which strongly indicates a local software or driver problem rather than a cabling fault. Final Answer: The PC host PC1 has the TCP/IP protocol stack correctly installed and functioning on the local machine.

Question 3

Users on the Internet must be able to access web pages hosted on Web Server 2, which has the internal address 192.168.10.20 behind router R1. What must be configured so that Internet users can reach this server?

Accepted Answer

Introduction / Context: In many enterprise and small business networks, internal servers use private IP addresses that are not routable on the public Internet. To allow Internet users to reach such servers, routers at the edge of the network use Network Address Translation. Specifically, static NAT or port forwarding can map a public IP address to a private server address, enabling external access while preserving internal addressing schemes. Given Data / Assumptions: Web Server 2 has an internal private IP, 192.168.10.20. Router R1 connects the internal network to an external network 209.165.100.0/24. External users must access the web server over the Internet. We assume proper routing to the public address already exists in the service provider network. Concept / Approach: Private addresses, such as those in the 192.168.0.0/16 range, cannot be routed across the public Internet. When external clients send traffic to a public IP owned by the organization, R1 must translate that public address to the private address of the internal web server and vice versa. This is achieved with static NAT configuration on R1. DNS may map a domain name to the public IP, but the key step is configuring NAT to perform the translation between public and private addresses. Step-by-Step Solution: 1. Recognize that 192.168.10.20 is a private IP address that is not reachable directly from the Internet.2. Identify that the organization has public addresses in the 209.165.100.0/24 network.3. To allow external users to reach the internal server, map one public address from 209.165.100.0/24 to 192.168.10.20 on router R1 using NAT.4. Configure appropriate static NAT or port forwarding rules on R1.5. With NAT in place, external users connect to the public IP, and R1 translates traffic to and from the internal server. Verification / Alternative check: You can verify the configuration by attempting to access the web server from an external test client using the public IP. On R1, commands such as show ip nat translations can confirm that translations are occurring. A successful web page load demonstrates that NAT is working and that routing and firewall rules permit the connection. Why Other Options Are Wrong: Option A sets a route on R2, which does not solve the fundamental problem of translating between public and private addresses. Option B, configuring DNS on R2, might map a name to an IP, but without NAT the private address remains unreachable from the Internet. Option D incorrectly suggests assigning a public address directly to the internal server using DHCP; this is not typical in a private internal network behind a router and does not address translation requirements. Common Pitfalls: A common mistake is to rely only on DNS and forget that routing and addressing still require public reachability. Another pitfall is misconfiguring NAT so that the wrong internal address is mapped, leading to connection failures. Administrators must also remember to adjust firewall rules so that HTTP or HTTPS traffic is allowed to reach the translated server. Final Answer: On router R1, configure Network Address Translation (NAT) to translate a public address on the 209.165.100.0/24 network to 192.168.10.20.

Question 4

The router interface with IPv4 address 192.168.10.250 acts as the default gateway for both Web Server 2 and Host 1 on the same LAN. What is the correct subnet mask for this network?

Accepted Answer

Introduction / Context: Subnet masks define how host IP addresses are grouped into networks and broadcast domains. When multiple devices share the same default gateway on a simple LAN, they must be in the same IP subnet as the router interface. Choosing the correct subnet mask ensures that hosts correctly identify which addresses are local and which must be forwarded to the gateway. Given Data / Assumptions: Router interface address: 192.168.10.250. Web Server 2 and Host 1 use this address as their default gateway. All three devices are on the same physical LAN segment. We want a typical small office configuration without unusually small subnets. Concept / Approach: The address 192.168.10.250 is in a private Class C range. The most common subnet mask for such a network is 255.255.255.0, which yields 256 addresses (including network and broadcast). This mask allows host addresses from 192.168.10.1 through 192.168.10.254, so 192.168.10.250, Web Server 2, and Host 1 can all reside on the same subnet and share the same default gateway. More restrictive masks, such as 255.255.255.192 or 255.255.255.252, would divide the network into smaller ranges and could place the hosts and gateway into incompatible subnets if not carefully designed. Step-by-Step Solution: 1. Recognize that 192.168.10.250 is a typical gateway address near the top of the usable host range.2. With mask 255.255.255.0, the network is 192.168.10.0 and the broadcast address is 192.168.10.255.3. Valid host addresses in this subnet range from 192.168.10.1 to 192.168.10.254.4. Using this mask, both Web Server 2 and Host 1 can have 192.168.10.x addresses with 192.168.10.250 as their gateway.5. Therefore, 255.255.255.0 is the correct subnet mask for this simple LAN. Verification / Alternative check: To verify, you can compute the subnet boundaries for each candidate mask. For example, with 255.255.255.192, the block size in the last octet is 64, so the subnets are 192.168.10.0, .64, .128, and .192. The address 192.168.10.250 falls in the .192 subnet range, which only has host addresses .193 through .254. If hosts are not consistently assigned within the same range, they may not share the gateway correctly. The mask 255.255.255.0 avoids this complication and matches the usual design for such scenarios. Why Other Options Are Wrong: Option B, 255.255.255.192, creates smaller subnets and may separate hosts unnaturally. Option C, 255.255.255.250, is not a valid contiguous subnet mask. Option D, 255.255.255.252, allows only two usable host addresses, which is too small for one gateway and multiple hosts on a typical LAN. Common Pitfalls: Learners sometimes pick masks based only on the numeric pattern without checking whether all devices fall inside the same subnet. Another pitfall is misunderstanding valid subnet masks and proposing values that do not correspond to contiguous one bits followed by zeros in binary. Final Answer: 255.255.255.0

Question 5

In practical IPv6 deployments, encapsulating IPv6 packets inside IPv4 packets so they can cross an IPv4 network is known as what?

Accepted Answer

Introduction / Context: During the transition from IPv4 to IPv6, many networks must carry IPv6 traffic over existing IPv4 infrastructure. One common way to accomplish this is to encapsulate IPv6 packets inside IPv4 packets, allowing them to traverse IPv4-only segments without modification to the core network. Understanding the name and purpose of this technique is important for network design and certification exams. Given Data / Assumptions: We have both IPv4 and IPv6 networks in the overall topology. Some intermediate networks support only IPv4. IPv6 packets need to cross these IPv4-only sections. The question asks for the term used when IPv6 is encapsulated in IPv4. Concept / Approach: Encapsulating one protocol inside another for transport across an incompatible network is referred to as tunneling. In the context of IPv6, tunneling methods such as 6to4 or manually configured tunnels wrap IPv6 packets inside IPv4 headers. The outer IPv4 header provides addressing across the IPv4 network, while the inner IPv6 packet is delivered unchanged once the tunnel is terminated. This is distinct from routing, which simply forwards packets, and from NAT, which modifies addresses. Step-by-Step Solution: 1. Identify the problem: IPv6 packets must travel across an IPv4-only network.2. Recognize that the solution is to place the IPv6 packet inside an IPv4 packet.3. This process of encapsulating one protocol packet inside another is known as tunneling.4. The tunnel endpoints add and remove the outer IPv4 header.5. Therefore, the correct term for this method is tunneling. Verification / Alternative check: IPv6 transition documents and RFCs describe several tunneling mechanisms that use this principle. Examples include manually configured tunnels, 6to4, and ISATAP. These all rely on encapsulating IPv6 inside IPv4, which clearly matches the definition of a tunnel rather than simple routing or address translation. Why Other Options Are Wrong: Option B, hashing, refers to generating a fixed-size value from input data, often used in security or load balancing, not in this encapsulation context. Option C, routing, describes the generic process of forwarding packets based on destination addresses without encapsulating another protocol inside. Option D, Network Address Translation, modifies addresses and sometimes ports but does not describe the process of wrapping IPv6 inside IPv4 packets end to end. Common Pitfalls: Students sometimes confuse tunneling with NAT because both appear in transition scenarios. However, tunneling preserves the original packet unchanged inside an outer header, while NAT alters address information. Another mistake is thinking that routing by itself can overcome protocol incompatibility; in reality, encapsulation is needed when the core cannot understand the inner protocol. Final Answer: Tunneling

Question 6

In standard Ethernet, which set of fields belongs to the data link layer portion of a frame header and trailer?

Accepted Answer

Introduction / Context: Ethernet frames are the fundamental units of data at the data link layer on most local area networks. Understanding which fields belong to the Ethernet frame header and trailer helps learners distinguish between layer 2 and higher layer information and correctly interpret packet captures and network diagrams. Given Data / Assumptions: We are dealing with standard Ethernet framing as defined for IEEE 802.3. We focus on the fields used at the data link layer, not the network or transport layers. The question asks which group of fields makes up the relevant portion of the Ethernet frame that carries user data and error checking information. Higher layer fields such as IP addresses and port numbers are not part of the data link layer frame header or trailer. Concept / Approach: An Ethernet frame at the data link layer typically includes the destination MAC address, source MAC address, an optional 802.1Q tag, a Length or Type field, the payload data, and a Frame Check Sequence for error detection. The MAC addresses identify the sending and receiving network interfaces on the local link, while the Length or Type field tells the receiver what protocol is encapsulated, such as IPv4 or IPv6. The FCS provides a cyclic redundancy check to detect transmission errors. IP addresses and transport layer ports are encapsulated inside this payload and belong to higher layers. Step-by-Step Solution: 1. Recall the structure of an Ethernet frame: destination MAC, source MAC, Length/Type, data, and FCS.2. Option A lists exactly these fields, describing a complete data link layer frame header, payload, and trailer.3. Option B contains Source IP, Destination IP, and TCP port, which belong to network and transport layers.4. Option C mixes preamble and start-of-frame delimiter with Source IP, but the IP field is not part of the data link header.5. Option D describes a mix of VLAN ID and higher layer fields, which does not match the core Ethernet data link structure described in the question. Verification / Alternative check: Network references and protocol analyzers such as Wireshark show the Ethernet frame header fields clearly separated from encapsulated IP and TCP or UDP headers. Inspecting real traffic confirms that the data link layer fields include MAC addresses, Length/Type, payload, and FCS, matching option A. Why Other Options Are Wrong: Option B focuses entirely on layer 3 and layer 4 information, which sits inside the Ethernet payload, not in the Ethernet frame header or trailer. Option C introduces IP information into what should be purely layer 2, and the combination listed is not the canonical Ethernet frame layout. Option D includes VLAN ID and transport layer details but omits the essential MAC and FCS fields defined for the data link layer. Common Pitfalls: It is easy to confuse the layered model when reading packet captures, because tools display all headers together. Students sometimes think that IP addresses or port numbers are part of the Ethernet frame itself rather than payload data. Remember that the data link layer is concerned with local delivery on a single link, using MAC addresses and FCS, while IP and TCP or UDP operate at higher layers. Final Answer: Destination MAC, Source MAC, Length/Type, Data, Frame Check Sequence (FCS)

Question 7

After obtaining an IPv4 address from a DHCP server, how does a DHCP client ensure that no other computer is already using that address on the local network?

Accepted Answer

Introduction / Context: IP address conflicts can cause serious connectivity problems on a local area network. DHCP includes mechanisms to minimize the risk of two hosts using the same IP address. One of these mechanisms involves the use of ARP messages at the data link layer to verify that an address is not already in use before a client begins normal communication with that address. Given Data / Assumptions: A client has just received an IPv4 address and related configuration from a DHCP server. The address is within a shared broadcast domain, such as an Ethernet LAN. The client must check for conflicts before fully using this address. We are considering standard behavior described in DHCP and ARP practices. Concept / Approach: A gratuitous ARP is an ARP request or reply that announces a host's own IP-to-MAC mapping, often sent without being explicitly requested by another device. When a client sends a gratuitous ARP for its newly assigned IP, it both claims the address and checks for duplicates. If another device is already using that IP, it may respond, alerting the client to a conflict. This technique goes beyond simply pinging the address because ARP operates at the link layer, where conflicts are first detected. Step-by-Step Solution: 1. The DHCP client receives an address and configuration from the DHCP server.2. Before fully adopting the address, the client sends a gratuitous ARP on the local network that includes its own IP and MAC.3. This ARP is broadcast to all devices in the LAN segment.4. If no other device responds with a conflicting ARP, the client assumes that the address is unique and safe to use.5. If a response indicating a duplicate appears, the client or server can take steps to resolve the conflict, such as selecting a different address. Verification / Alternative check: Packet captures on a DHCP-enabled network show clients sending ARP probes or gratuitous ARPs after address assignment. Observing this traffic confirms that the client actively tests for conflicts as part of its startup behavior. Some systems also log IP conflict messages if a duplicate is detected, which further demonstrates the importance of gratuitous ARP. Why Other Options Are Wrong: Option A mentions acknowledging a TCP segment, which is unrelated to address conflict detection at startup. Option B suggests pinging the address, but ICMP echo is less reliable than ARP for detecting conflicts and is not the standard method used by DHCP clients. Option C refers to Proxy ARP, where a router answers ARP requests on behalf of remote hosts; this is not how a client checks its own address. Common Pitfalls: A common misconception is that DHCP alone prevents conflicts through careful lease management. In reality, ARP-based checks provide an additional safety net. Another pitfall is not recognizing gratuitous ARP packets in network traces and missing evidence of conflict detection mechanisms. Final Answer: Broadcast a gratuitous ARP announcing its own IP-to-MAC mapping and checking for conflicts.

Question 8

In binary notation, which pattern represents the first octet of a Class B IPv4 network address?

Accepted Answer

Introduction / Context: Traditional classful IPv4 addressing divides addresses into classes A, B, C, and others based on the leading bits of the first octet. Although modern networks use classless addressing, understanding these patterns remains useful for historical questions and for quickly recognizing approximate address ranges. Given Data / Assumptions: We are considering the first octet of an IPv4 address in binary. Class B addresses historically covered decimal values from 128 to 191. We must express this range as a bit pattern for the first eight bits. The x characters in the pattern represent either 0 or 1. Concept / Approach: An 8-bit octet can represent values from 0 to 255. Class B addresses start at 128 and end at 191. In binary, 128 is 10000000 and 191 is 10111111. The common prefix for all values in this range is 10, meaning that all Class B addresses begin with the bits 10 followed by six bits that can vary. Therefore, the general pattern for a Class B first octet is 10xxxxxx. Step-by-Step Solution: 1. Recall the decimal range for Class B first octet values: 128 through 191.2. Convert 128 to binary: 10000000.3. Convert 191 to binary: 10111111.4. Observe that both values share the prefix bits 10 in the most significant positions.5. Replace the remaining six bits with x characters to indicate any combination, giving 10xxxxxx. Verification / Alternative check: You can quickly test a few values within the range, such as 130 or 150, and confirm that their binary representations also start with 10. Checking values outside the range, such as 110 or 200, shows different prefixes, confirming that only Class B addresses share the 10xxxxxx pattern. Why Other Options Are Wrong: Option A, 01xxxxxx, corresponds to decimal values roughly from 64 to 127, which are not Class B. Option B, 0xxxxxxx, covers values from 0 to 127 and mainly includes Class A ranges. Option D, 110xxxxx, describes the prefix for Class C addresses, which start at 192. Common Pitfalls: Learners sometimes reverse the mapping of bit patterns to classes or confuse Class B and Class C prefixes because both begin with 1. Remember the specific prefixes: 0 for Class A, 10 for Class B, and 110 for Class C in the first octet. Final Answer: 10xxxxxx

Question 9

Which mechanism allows a router to respond to an ARP request on behalf of a remote host, effectively providing its own MAC address instead?

Accepted Answer

Introduction / Context: Address Resolution Protocol is used on broadcast networks to map IP addresses to MAC addresses. Sometimes, a host may attempt to reach another IP address that is not on the same subnet. In certain legacy or special configurations, routers can assist by answering ARP requests on behalf of remote hosts, providing their own MAC address so that traffic is forwarded correctly. Given Data / Assumptions: A router is connected to multiple networks or subnets. A host sends an ARP request for an IP address that is not on its local subnet. The router has a route to the requested IP address on a different network. The question asks which mechanism lets the router answer the ARP request for that remote host. Concept / Approach: Proxy ARP enables a router to reply to ARP requests intended for a host that is not directly reachable on the local network. The router answers with its own MAC address, effectively telling the requesting host to send the traffic to the router. The router then forwards the packet toward the true destination using its routing table. This provides a form of transparent subnetting, though it is generally avoided in modern designs because it can introduce confusion. Step-by-Step Solution: 1. A host sends an ARP request for a destination IP that lies on a remote subnet.2. The router recognizes that it has a route to that IP and that Proxy ARP is enabled on the interface receiving the ARP.3. Instead of ignoring the ARP, the router replies with its own MAC address corresponding to that IP.4. The host updates its ARP cache, associating the destination IP with the router's MAC.5. The host then sends packets for that destination to the router, which forwards them appropriately. Verification / Alternative check: Network documentation and configuration guides describe Proxy ARP as this specific feature. Packet captures on an interface with Proxy ARP enabled will show the router replying to ARP requests for IP addresses that are not assigned to the local segment, confirming the behavior. Why Other Options Are Wrong: Option A, Gateway DP, is not a standard networking mechanism. Option B, Reverse ARP, is used by a host to learn its own IP address from a server based on its MAC, not to answer ARP for remote hosts. Option D, Inverse ARP, is used in Frame Relay and similar technologies to discover network layer addresses corresponding to known virtual circuit identifiers, not to respond to Ethernet ARP queries. Common Pitfalls: Some learners confuse Proxy ARP with NAT or routing in general. Proxy ARP works specifically at the ARP layer by manipulating MAC address responses. It does not change IP addresses or provide broad routing functionality by itself; it simply allows a router to masquerade as another host in ARP responses. Final Answer: Proxy ARP

Question 10

In IPv4 networking, what is the special address 127.0.0.1 commonly used for?

Accepted Answer

Introduction / Context: Some IP addresses are reserved for special purposes and are not used as normal host addresses on a network. One of the most important special addresses in IPv4 is 127.0.0.1. Understanding its role helps with troubleshooting and clarifies why certain pings never leave the local computer. Given Data / Assumptions: We are working with IPv4 addresses. The address in question is 127.0.0.1. Network devices and operating systems implement this address consistently. The question asks for the common use of this specific address. Concept / Approach: Address 127.0.0.1 is part of the 127.0.0.0/8 loopback block, which is reserved for internal host communication. When an application sends traffic to 127.0.0.1, the IP stack routes the packet back to itself without placing it on the network. This allows administrators and applications to test the local TCP/IP stack, services, and interprocess communication without involving external devices. Step-by-Step Solution: 1. Recognize that 127.0.0.1 is defined as the standard loopback address in IPv4.2. Traffic sent to this address is processed internally by the host's IP stack.3. No frames are placed on the physical network interface for this destination.4. Administrators often ping 127.0.0.1 to verify that TCP/IP is installed and functioning correctly on the local machine.5. Therefore, the correct description is that 127.0.0.1 is used as a loopback address for local testing. Verification / Alternative check: If you disconnect the network cable or disable the wireless adapter and ping 127.0.0.1, the ping still succeeds if the TCP/IP stack is operational. This demonstrates that the traffic never requires network hardware. Documentation and standards also state that the 127.0.0.0 range is reserved for loopback. Why Other Options Are Wrong: Option A is incorrect because default gateways are usually addresses on the local subnet such as 192.168.1.1, not 127.0.0.1. Option B is wrong since 127.0.0.1 is not routable on the Internet and cannot test external connectivity. Option D is incorrect because broadcast addresses depend on the subnet mask and are typically of the form network with all host bits set to one, such as 192.168.1.255. Common Pitfalls: Sometimes students mistakenly believe that a successful ping to 127.0.0.1 proves that the network card, cable, or switch are functioning. In reality, it only proves that the local IP stack is working. Additional tests to the host's own IP address, the default gateway, and remote systems are needed to verify full network connectivity. Final Answer: As the loopback address used to test the local TCP/IP stack on the host.

Question 11

Which of the following IPv4 address ranges is the Class A private address space defined by RFC 1918?

Accepted Answer

Introduction / Context: Private IPv4 address ranges are reserved for use inside organizations and are not routable on the public Internet. They are defined in RFC 1918 and are commonly used in home, enterprise, and data center networks. Recognizing which ranges belong to which historical classes helps in configuring routers, firewalls, and NAT policies. Given Data / Assumptions: We focus on the Class A private range as defined by RFC 1918. Several candidate address ranges are listed, some of which are other private ranges or special-purpose blocks. Only one option represents the correct Class A private address space. We assume standard IPv4 and traditional classful terminology. Concept / Approach: RFC 1918 specifies three private address ranges: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. Historically, 10.0.0.0/8 is associated with Class A, 172.16.0.0/12 with Class B, and 192.168.0.0/16 with Class C. The 10.0.0.0/8 range covers 10.0.0.0 through 10.255.255.255, making it the correct Class A private range. Step-by-Step Solution: 1. Recall that RFC 1918 defines 10.0.0.0/8 as a private block.2. The /8 mask means that the first octet, 10, is the network portion and the remaining three octets can vary.3. This yields a range from 10.0.0.0 up to 10.255.255.255.4. Compare this with the options and identify the range that exactly matches 10.0.0.0 – 10.255.255.255.5. Therefore, option A is the correct Class A private address space. Verification / Alternative check: Consulting documentation or networking references on RFC 1918 confirms that 10.0.0.0/8 is reserved for private use and is often used in large enterprise environments due to its large number of available addresses. Why Other Options Are Wrong: Option B, 172.16.0.0 – 172.31.255.255, is the Class B style private range defined by RFC 1918. Option C, 192.168.0.0 – 192.168.255.255, is associated with the Class C style private range. Option D, 169.254.0.0 – 169.254.255.255, is the link-local range used for automatic private IP addressing when DHCP fails and is not part of the RFC 1918 private ranges. Common Pitfalls: Learners sometimes mix up the different private ranges or think that any non-routable address is part of RFC 1918. It is important to memorize the three blocks and associate them with their typical usage sizes: very large 10.0.0.0/8, medium 172.16.0.0/12, and smaller 192.168.0.0/16. Final Answer: 10.0.0.0 – 10.255.255.255

Question 12

Which IPv4 address range is the Class B private address space defined for internal use?

Accepted Answer

Introduction / Context: Along with the Class A and Class C style private ranges, IPv4 defines a Class B style private address block intended for internal network use. Knowing this range helps network administrators design addressing schemes and recognize non-routable internal addresses when troubleshooting. Given Data / Assumptions: We focus on the Class B style private address block defined by RFC 1918. Several ranges are provided, including other private ranges and a generic class range. Only one option is the correct private Class B style range. We assume standard IPv4 and traditional terminology. Concept / Approach: RFC 1918 defines 172.16.0.0/12 as a private address space. A /12 mask covers 172.16.0.0 through 172.31.255.255. This range sits between the smaller 192.168.0.0/16 and the larger 10.0.0.0/8 private blocks and is often associated with Class B sized networks in older terminology. Step-by-Step Solution: 1. Recall that the Class B style private range in RFC 1918 is 172.16.0.0/12.2. The /12 mask means the first twelve bits are fixed, which corresponds to addresses from 172.16.0.0 to 172.31.255.255.3. Compare this range against the options provided.4. Option A matches exactly 172.16.0.0 – 172.31.255.255.5. Therefore, option A is the correct Class B private address space. Verification / Alternative check: Network textbooks and RFC 1918 itself list this range explicitly. It is commonly used in medium to large organizations that want more addresses than are available in 192.168.0.0/16 but do not require the entire 10.0.0.0/8 block. Why Other Options Are Wrong: Option B, 10.0.0.0 – 10.255.255.255, is the Class A style private range. Option C, 192.168.0.0 – 192.168.255.255, is the Class C style private range. Option D, 128.0.0.0 – 191.255.255.255, represents the entire historical Class B public space, not the specific private subset defined by RFC 1918. Common Pitfalls: People sometimes assume that any 172.x.y.z address is private, but only the subset from 172.16.0.0 to 172.31.255.255 is defined as private. Addresses like 172.32.0.1 are public and routable. Memorizing the exact start and end of the private range avoids such mistakes. Final Answer: 172.16.0.0 – 172.31.255.255

Question 13

In IPv4 addressing, which range represents the Class C private IP address space as defined by RFC 1918?

Accepted Answer

Introduction / Context: This question tests your understanding of private IP address ranges in IPv4, specifically the block commonly referred to as the Class C private address space. Knowing which ranges are reserved for private use is essential for designing internal networks, configuring Network Address Translation, and avoiding conflicts with public Internet address space. Given Data / Assumptions: - We are working with IPv4, not IPv6. - The question refers to the historic classful terminology, but modern networks still use the same private ranges defined by RFC 1918. - RFC 1918 defines three private ranges: one traditionally associated with Class A, one with Class B, and one with Class C. - We must identify the range that corresponds to the Class C style private block. Concept / Approach: RFC 1918 defines the following private IPv4 address ranges: 10.0.0.0/8 (commonly called the Class A private range). 172.16.0.0/12 (commonly called the Class B private range). 192.168.0.0/16 (commonly called the Class C private range). The Class C terminology comes from the original classful idea that Class C networks start at 192.0.0.0 and run through 223.255.255.255, with many small networks. The reserved private part of this spectrum is 192.168.0.0 to 192.168.255.255. Step-by-Step Solution: Step 1: Recall the three RFC 1918 private blocks: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16.Step 2: Associate 10.0.0.0/8 with the traditional Class A space because it covers the entire first octet 10.Step 3: Associate 172.16.0.0/12 with the traditional Class B space because it spans a range of contiguous Class B networks.Step 4: Recognize that 192.168.0.0/16 is the private range that sits in the historic Class C portion of the IPv4 address space.Step 5: Translate the 192.168.0.0/16 prefix into the explicit range 192.168.0.0 through 192.168.255.255. Verification / Alternative check: You can verify this by thinking about common home and small office routers, which often default to networks such as 192.168.0.0/24 or 192.168.1.0/24. These are subnets of the larger 192.168.0.0/16 private block. In contrast, large enterprise networks often use 10.0.0.0/8 because it offers a very large internal address space, and some organizations use the 172.16.0.0/12 range for medium sized deployments. Why Other Options Are Wrong: 10.0.0.0/8 is a private range but is the Class A style private block, not the Class C space. 172.16.0.0/12 is the Class B style private block. 169.254.0.0/16 is used for Automatic Private IP Addressing when Dynamic Host Configuration Protocol is unavailable and is not an RFC 1918 private range in the same sense. The 224.0.0.0 to 239.255.255.255 block is Class D, reserved for multicast, not for private unicast addressing. Common Pitfalls: A common mistake is to assume that any address starting with 192 is private or that 169.254.0.0/16 is part of RFC 1918. Another pitfall is to forget the exact boundaries of the 172.16.0.0/12 range and incorrectly label it as Class C style. Remembering the three canonical private ranges exactly as 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 helps avoid these errors. Final Answer: The Class C private IP address space is 192.168.0.0 to 192.168.255.255, which is the 192.168.0.0/16 block.

Question 14

In a typical TCP/IP network, what are the primary functions of the DNS and DHCP services and how do they complement each other?

Accepted Answer

Introduction / Context: This conceptual question checks whether you understand the distinct but complementary roles of Domain Name System and Dynamic Host Configuration Protocol in an IP network. These two services are fundamental building blocks in almost every modern enterprise and home network. Misunderstanding their roles can lead to configuration errors and troubleshooting difficulties. Given Data / Assumptions: - We are working in a standard IPv4 or IPv6 TCP/IP environment. - Client devices need IP addresses and other settings to communicate. - Users prefer to connect to resources using names such as server1.example.com rather than numerical IP addresses. - DNS and DHCP are both present somewhere in the network and may run on the same or different servers. Concept / Approach: Dynamic Host Configuration Protocol is designed to automate the process of assigning IP configuration parameters to clients. This typically includes the IP address, subnet mask, default gateway, and DNS server addresses, and may include many other options. Domain Name System is a distributed directory that maps human friendly hostnames to IP addresses and sometimes performs reverse lookups from IP to name. These services complement one another: DHCP tells clients which DNS servers to use, and DNS allows clients to translate names into usable IP addresses for communication. Step-by-Step Solution: Step 1: Identify DHCP as the protocol that dynamically assigns IP addresses and related configuration when a device joins the network.Step 2: Recognize that DNS is used to resolve hostnames, such as www.example.com, into IP addresses so that applications can connect.Step 3: Note that DHCP usually provides the addresses of DNS servers to clients so that they know where to send queries.Step 4: Understand that DNS does not normally assign addresses and DHCP does not resolve names, so their functions are separate but coordinated.Step 5: Choose the option that clearly states DNS resolves hostnames to IP addresses and DHCP automatically assigns IP configuration to clients. Verification / Alternative check: You can verify this understanding by thinking about a typical client configuration screen. When set to obtain an IP address automatically, the client sends DHCP messages to get an address and other parameters. One of those parameters is the DNS server address. After this step, when the user opens a browser and types a name, the client sends a DNS query to the configured DNS server, which returns an IP address. This sequence confirms the roles of DHCP and DNS. Why Other Options Are Wrong: Any option claiming that DNS assigns IP addresses reverses the roles of the protocols. DNS does not distribute addresses to clients. Options suggesting that DNS forwards packets or encrypts traffic confuse it with routing or security protocols. DHCP does not manage default gateway redundancy or user authentication by itself. Likewise, DNS does not manage MAC address tables, which are part of switching, and DHCP does not discover switch hardware. Common Pitfalls: A frequent mistake is to assume that DHCP is only about IP addresses and to forget that it also distributes DNS and default gateway information. Another pitfall is to think of DNS merely as a static hosts file substitute, ignoring the fact that it is a full hierarchical distributed database. Keeping their roles straight helps speed up troubleshooting when name resolution works by IP address but fails by hostname. Final Answer: DNS resolves hostnames to IP addresses, while DHCP automatically assigns IP configuration such as address, subnet mask, gateway, and DNS servers to clients.

Question 15

On a traditional IPv4 point-to-point WAN link, which subnet mask is typically used to minimize wasted IP addresses while still providing two usable host addresses?

Accepted Answer

Introduction / Context: This question focuses on efficient subnetting for point-to-point Wide Area Network links. On such links you typically need only two usable addresses, one for each end of the link. Using a subnet that is larger than necessary wastes addresses. The question reflects the traditional design approach used in many certification exams and production networks. Given Data / Assumptions: - The medium is a point-to-point IPv4 WAN link between two routers. - Only two usable host addresses are required for the link endpoints. - We want to reduce wastage of IP addresses while remaining compatible with common router configurations and older textbooks. - Modern /31 usage is known but is not assumed as the default in many exam oriented questions. Concept / Approach: In IPv4, a subnet must have enough addresses for the required hosts plus two reserved addresses for the network and broadcast in traditional designs. For a link that requires only two hosts, the smallest traditional subnet that supports this is a /30 network. A /30 has 4 total addresses: one network address, two usable host addresses, and one broadcast address. A /31 has only 2 addresses and is usable on some equipment under specific conditions, but historically and in many exam contexts /30 is the expected answer. Step-by-Step Solution: Step 1: Recall the basic subnet formula that total addresses per subnet equal 2^(number of host bits).Step 2: Recognize that a /30 prefix means 32 - 30 = 2 host bits, giving 2^2 = 4 total addresses in the subnet.Step 3: Subtract 2 reserved addresses (network and broadcast) from the 4 total addresses, leaving 2 usable host addresses, which is exactly what a point-to-point link needs.Step 4: Compare with /29, which gives 8 addresses and 6 usable hosts, more than required, and /28, which wastes even more.Step 5: Understand that /31 can be used on some routers but historically was not the default and is often not considered in basic exam questions, so /30 is selected. Verification / Alternative check: You can verify by writing a simple example such as 192.0.2.0/30. The addresses in this subnet are 192.0.2.0 (network), 192.0.2.1 (host), 192.0.2.2 (host), and 192.0.2.3 (broadcast). The two usable addresses 192.0.2.1 and 192.0.2.2 are sufficient for the two ends of a point-to-point link, and no additional hosts are required. Why Other Options Are Wrong: A /28 subnet provides 16 total addresses and 14 usable host addresses, which is more than needed and wastes address space. A /29 subnet provides 8 total addresses and 6 usable hosts, which still wastes four host addresses. A /31 subnet can be used for special point-to-point designs under RFC 3021, but many basic configurations and exam questions still focus on /30 as the standard answer. A /32 represents a single host route, not a subnet for two routers to share. Common Pitfalls: One common pitfall is to forget to subtract the network and broadcast addresses from the total host count. Another is to jump directly to /29 or /28 because they seem small, without formally calculating the usable hosts. Finally, some learners assume that /31 is always valid for any point-to-point link without checking hardware or exam context, which may lead to incorrect answers in traditional certification questions. Final Answer: The typical mask used to minimize wasted addresses on a traditional point-to-point WAN link is /30, which is 255.255.255.252.

Question 16

For a host with IPv4 address 200.10.5.68/28, what is the correct subnetwork (network) address for the subnet to which this host belongs?

Accepted Answer

Introduction / Context: This question checks your ability to determine the network address of a subnet when given a host IP address and a prefix length. This skill is fundamental in IPv4 subnetting, routing, and troubleshooting tasks, since routers and administrators think in terms of network addresses rather than individual hosts when designing and summarizing networks. Given Data / Assumptions: - Host IP address: 200.10.5.68. - Prefix length: /28, which corresponds to a subnet mask of 255.255.255.240. - We must find the network address of the subnet that contains this host. - The network address is the first address in the subnet and has all host bits set to 0. Concept / Approach: A /28 prefix means that 28 bits are used for the network portion, leaving 4 bits for the host portion in the last octet. Four host bits give 2^4 = 16 total addresses per subnet. Therefore, subnets in the last octet increment in blocks of 16. By identifying the block in which 68 falls, we can determine the correct network address. The valid blocks in the last octet are 0, 16, 32, 48, 64, 80, 96, and so on. Step-by-Step Solution: Step 1: Convert the /28 prefix to a subnet mask: 255.255.255.240. This confirms there are 16 addresses per subnet in the last octet.Step 2: Determine the block size in the last octet as 256 - 240 = 16.Step 3: List the subnet start values in the last octet: 0, 16, 32, 48, 64, 80, 96, and so on up to 240.Step 4: Identify where the host value 68 falls. It is greater than or equal to 64 and less than 80, so it belongs to the 64 to 79 range.Step 5: The network address for that range is 200.10.5.64, and the broadcast address is 200.10.5.79, with usable hosts from 200.10.5.65 to 200.10.5.78. Verification / Alternative check: You can verify by subtracting the nearest lower multiple of 16 from the host address last octet. Dividing 68 by 16 gives 4 with a remainder, and 4 * 16 = 64. This confirms that the network address is 200.10.5.64. Any host with a last octet between 64 and 79 inclusive would be in that same subnet under a /28 mask. Why Other Options Are Wrong: 200.10.5.56 would be the network address if the block size created a range that contained 56, but 56 is in the previous block (48 to 63), which does not include 68. 200.10.5.32 belongs to the 32 to 47 range, again not containing 68. 200.10.5.0 would be the network address for the first subnet only and does not contain 68. 200.10.5.80 is actually the start of the next subnet range (80 to 95) and is greater than the host address 68. Common Pitfalls: A common mistake is to pick the closest lower multiple of 8, 32, or another number without correctly calculating the block size from the mask. Another error is to confuse the broadcast address with the network address. Remember that the network address is always the lowest address in the block and has all host bits set to 0. Practicing with simple block size calculations helps reduce these errors in subnetting questions. Final Answer: The correct subnetwork address for host 200.10.5.68 with a /28 prefix is 200.10.5.64.

Question 17

What is the maximum number of usable host IP addresses that can be assigned on a local subnet using the subnet mask 255.255.255.224?

Accepted Answer

Introduction / Context: This question evaluates your ability to calculate the number of usable host IP addresses available in a subnet given its subnet mask. This is a basic but essential subnetting skill that you will use when planning VLANs, allocating addresses to departments, or designing WAN segments. Given Data / Assumptions: - The subnet mask is 255.255.255.224. - We are concerned with the maximum number of IP addresses that can be assigned to hosts on a single subnet using this mask. - Network and broadcast addresses cannot be assigned to hosts. - We are working in a traditional IPv4 context where each subnet reserves two addresses (network and broadcast). Concept / Approach: The subnet mask 255.255.255.224 corresponds to a /27 prefix, because the first 27 bits are set to 1. That leaves 5 bits for host addresses in the last octet (since 32 - 27 = 5). The total number of addresses in a subnet equals 2^(number of host bits). To find the number of usable host addresses, you subtract 2 from this value to account for the network address (all host bits 0) and the broadcast address (all host bits 1). Step-by-Step Solution: Step 1: Convert the subnet mask 255.255.255.224 to prefix notation: 224 in binary is 11100000, so we have 8 + 8 + 8 + 3 = 27 network bits, which is /27.Step 2: Calculate the number of host bits as 32 - 27 = 5.Step 3: Find the total number of addresses by computing 2^5 = 32.Step 4: Subtract 2 addresses for the network and broadcast addresses, leaving 32 - 2 = 30 usable host addresses.Step 5: Confirm that this count matches typical subnetting tables, where a /27 network supports 30 hosts. Verification / Alternative check: Many subnetting cheat sheets or memorized patterns show that common masks have known host capacities: /24 supports 254 hosts, /25 supports 126, /26 supports 62, and /27 supports 30. This pattern confirms that a /27 (255.255.255.224) subnet can indeed host 30 devices. You could also quickly verify by picking an example subnet, such as 192.168.1.32/27, and listing addresses from 192.168.1.32 to 192.168.1.63. The first is the network address, the last is broadcast, leaving 30 usable IPs in between. Why Other Options Are Wrong: 15 and 16 usable hosts correspond to a /28 mask (16 total addresses, 14 usable) or a miscalculation. 31 usable hosts would imply 33 total addresses, which does not align with any power of two. The option 32 usable host addresses incorrectly counts the network and broadcast addresses as assignable, which is not allowed in standard IPv4 subnetting practice. Common Pitfalls: A common error is to forget to subtract 2 for network and broadcast, resulting in the total number of addresses instead of usable hosts. Another pitfall is miscounting the number of host bits when converting from dotted decimal mask to prefix length. Practicing quick binary conversions for common masks like 224 (11100000) and 240 (11110000) helps avoid these mistakes. Final Answer: A subnet using the mask 255.255.255.224 has 30 usable host IP addresses.

Question 18

If you want to create 8 usable subnets from a Class C network using the subnet mask 255.255.255.224, which global configuration command must be enabled on a Cisco router to allow the use of all subnets, including subnet zero?

Accepted Answer

Introduction / Context: This question examines your knowledge of older Cisco IOS behavior regarding subnet zero and the configuration required to use all possible subnets in a classful network. Historically, some textbooks and default configurations discouraged the use of the first and last subnet, but modern best practice allows using all subnets to conserve address space, which requires a specific command on some platforms. Given Data / Assumptions: - The base network is a Class C network. - The subnet mask 255.255.255.224 is used, which is a /27 prefix and creates 8 possible subnets in a Class C range. - We want to use all 8 subnets, including subnet zero. - The question is about a Cisco router global configuration command that enables this behavior. Concept / Approach: With a Class C network and a /27 mask, the number of subnets is 2^(number of borrowed bits). Moving from /24 to /27 borrows 3 bits, so there are 2^3 = 8 possible subnets. Older documentation sometimes recommended not using subnet zero (all subnet bits 0) to avoid confusion, which would leave only 7 usable subnets. On Cisco routers, the global configuration command ip subnet-zero enables the use of subnet zero, allowing all 8 subnets to be used. Modern IOS versions usually have this enabled by default, but it remains an important exam concept. Step-by-Step Solution: Step 1: Recognize that 255.255.255.224 corresponds to /27 and creates 8 subnets in a Class C network.Step 2: Recall that without subnet zero, only 7 of the 8 subnets would be considered usable.Step 3: Identify that Cisco introduced the ip subnet-zero command so that administrators can use the first subnet (subnet zero) just like other subnets.Step 4: Note that ip classless is related to classless routing behavior, not specifically to allowing subnet zero.Step 5: Conclude that Router(config)#ip subnet-zero is the required command to allow all 8 subnets. Verification / Alternative check: You can verify by reviewing sample configurations in Cisco training material. For example, if you have a 192.168.1.0/24 network and you apply a /27 mask, the subnets start at 192.168.1.0, 192.168.1.32, 192.168.1.64, and so on. Without ip subnet-zero, some older systems would consider 192.168.1.0/27 off limits, but with ip subnet-zero enabled, it becomes a normal usable subnet, increasing efficiency. Why Other Options Are Wrong: The ip classless command affects how the router handles routes that do not exactly match classful boundaries but does not control use of subnet zero. The ip version 6 command is not a valid Cisco global command in this form and relates to IPv6, not IPv4 subnetting. The ip unnumbered command is used to borrow an IP address from another interface, not to control subnet usage. Disabling IP routing would break the router function entirely and is unrelated to subnet allocation. Common Pitfalls: Students sometimes confuse ip classless with ip subnet-zero because both refer to classful behavior. Another pitfall is to assume that subnet zero is always forbidden, which is outdated. Modern designs routinely use subnet zero, and many IOS releases enable ip subnet-zero by default, but exams can still ask about the command explicitly, so it is important to recognize it. Final Answer: The required command is Router(config)#ip subnet-zero, which allows all 8 subnets, including subnet zero, to be used.

Question 19

To test the local TCP/IP protocol stack on your own host without sending traffic onto the physical network, which IP address should you ping?

Accepted Answer

Introduction / Context: This question tests your knowledge of the special loopback address range used to verify the health of the local TCP/IP stack. Pinging a loopback address is a common first step when troubleshooting network issues because it helps distinguish between local protocol problems and external connectivity problems. Given Data / Assumptions: - You want to test the IP stack on the local host only. - You do not want or need to test the network interface card, cabling, or any external network devices. - You will use the ping utility to send ICMP echo requests. - The question asks for a specific IP address to ping, not just a range. Concept / Approach: The entire 127.0.0.0/8 block is reserved for loopback addresses in IPv4. Any packet sent to an address in this range is handled internally by the local operating system and never placed onto the physical network. The most commonly used loopback address is 127.0.0.1. A successful ping to 127.0.0.1 confirms that the local TCP/IP stack is installed and functioning correctly. It does not, however, guarantee that the network card, drivers, or external path are working. Step-by-Step Solution: Step 1: Recall that loopback testing in IPv4 uses addresses in the 127.0.0.0/8 range.Step 2: Identify 127.0.0.1 as the conventional loopback address used on almost all systems.Step 3: Understand that pinging 127.0.0.1 exercises the IP stack and ICMP handling on the local host but does not leave the host.Step 4: Recognize that 127.0.0.0 and 127.0.0.255 have special roles (network and broadcast) and are not typically used in simple tests.Step 5: Choose 127.0.0.1 as the correct answer. Verification / Alternative check: In practice, if you suspect a network problem, you can run three basic tests: ping 127.0.0.1 to verify the local stack, ping your own IP address to verify local binding to the network interface, and ping the default gateway to check external connectivity. The first of these is always 127.0.0.1, which underscores its role as the standard loopback address. Why Other Options Are Wrong: 127.0.0.0 is the network address of the loopback block, not the conventional host address used for testing. 1.0.0.127 is a completely different public address and is not reserved for loopback; pinging it would attempt to reach a remote host. 127.0.0.255 is a broadcast address within the loopback range and is not normally used for a simple stack test. The address 0.0.0.0 has special meanings in routing and configuration but is not a valid ping destination for loopback testing. Common Pitfalls: Some learners mistakenly assume that any 127.x.x.x address is acceptable and forget that 127.0.0.1 is the standard used in documentation and troubleshooting examples. Another pitfall is to confuse loopback testing with testing the local interface IP address, which is a different step. Distinguishing between these tests helps diagnose whether an issue lies in the TCP/IP stack, the network adapter, or the external network. Final Answer: To test the local TCP/IP stack, you should ping the loopback address 127.0.0.1.

Question 20

When advertising a summarized route in IPv4, which network address from the block is used as the summary (aggregate) address?

Accepted Answer

Introduction / Context: This question tests your understanding of route summarization, also called route aggregation, in IPv4 networks. When you summarize multiple contiguous networks into a single advertisement, you must choose a single network address and prefix length to represent the entire block. Knowing which specific address is used as the summary is key to configuring correct static summaries and interpreting dynamic protocol summaries. Given Data / Assumptions: - We are considering a group of contiguous networks that can be summarized by a single prefix. - The networks share a common set of high order bits and differ only in the host or lower order network bits. - The question asks which of the networks in the block is used as the advertised summary address. - Standard classless addressing and summarization rules apply. Concept / Approach: When summarizing, you examine the binary representations of the network addresses and identify the longest common prefix that covers all networks in the block. The summary address is created by taking that common prefix and setting all remaining bits to zero. This process always yields the numerically lowest network address in the range. Therefore, the summary address is the first network in the block, not the last or a middle one. Step-by-Step Solution: Step 1: Consider a simple example: summarizing 192.168.16.0/24 through 192.168.19.0/24 into a single route.Step 2: Convert the varying octet (the third octet) to binary: 16, 17, 18, and 19 correspond to 00010000, 00010001, 00010010, 00010011.Step 3: Find the longest sequence of common high order bits among these values; for example, they may share the first 6 bits, creating a /22 summary.Step 4: Construct the summary address by keeping the shared bits and setting all remaining bits to 0, which yields 192.168.16.0/22.Step 5: Observe that this summary network corresponds to the first network in the contiguous block (192.168.16.0), confirming that the first network is used as the summary address. Verification / Alternative check: Anytime you perform summarization by hand, you can confirm the result by listing the numerical range of addresses covered by the chosen summary. The lower bound of that range will always be the smallest network address in the group. If you attempted to use the last network as the summary base, the binary representation with zeros in the host bits would usually fall below the range you intend to cover, or with different prefix lengths would fail to encapsulate the entire block cleanly. Why Other Options Are Wrong: The last network address in the block does not become the summary base, because setting host bits to zero on that address will either overshoot or misalign the summary range. The next to last and second network options are arbitrary and do not follow the binary common-prefix rule. The idea of using any odd-numbered network is incorrect and has no basis in summarization algorithms. Common Pitfalls: One common error is to confuse the highest network in the block with the summary address because it feels intuitive to describe summaries from the top down. Another is to skip the binary analysis and rely on mental shortcuts that fail when the block size is not a power of two aligned on convenient boundaries. Always remember that summary addresses are based on common high order bits, which naturally produces the numerically lowest network in the range. Final Answer: The summary route uses the first network address in the block as the advertised summary address.

CISCO Certification Questions