Question 1

Subnet range calculation (IPv4 /26): Given IP address 192.168.168.188 with subnet mask 255.255.255.192, what is the valid host range for that subnet (exclude network and broadcast addresses)?

Accepted Answer

Introduction / Context: Quickly determining the host range of a subnet is a core networking skill. The mask 255.255.255.192 corresponds to a /26, which creates blocks of 64 addresses. We must locate the correct block containing 192.168.168.188 and then exclude the network and broadcast addresses to get the usable host range. Given Data / Assumptions: IP: 192.168.168.188 Mask: 255.255.255.192 (that is, /26) We want the usable host range only. Concept / Approach: With /26, the increment (block size) is 64 in the last octet because 256 - 192 = 64. Subnets therefore start at .0, .64, .128, .192 within the /24 boundary. Determine which block contains .188, then compute network (.128), broadcast (.191), and the host span between them. Step-by-Step Solution: Compute block size: 256 - 192 = 64.List boundaries: .0–.63, .64–.127, .128–.191, .192–.255.Since 188 lies in .128–.191, network = .128 and broadcast = .191.Usable hosts run from .129 through .190 (inclusive). Verification / Alternative check: Binary math confirms: /26 leaves 6 host bits → 2^6 = 64 addresses, subtract 2 for network/broadcast → 62 usable hosts, matching .129–.190 as 62 addresses. Why Other Options Are Wrong: .129–.191 includes the broadcast (invalid for hosts). .128–.190 includes the network address. .128–.192 mixes network and the next subnet’s network (.192). Common Pitfalls: Forgetting to subtract network and broadcast; miscomputing the increment as 32 instead of 64 for /26; ignoring that ranges reset at every block boundary. Final Answer: 192.168.168.129-190

Question 2

ICMP control messaging: Which protocol is responsible for sending a “destination network unknown” message back to the originating host when routing fails?

Accepted Answer

Introduction / Context: The Internet Control Message Protocol (ICMP) provides network-layer control and error messaging for IP. When a router cannot forward a packet due to an unreachable network or other conditions, ICMP communicates the error back to the sender, aiding troubleshooting and adaptive behavior. Given Data / Assumptions: The scenario is a routing failure where a destination network cannot be reached. We want the protocol that carries the error notification. This is independent of the transport layer protocol of the original traffic. Concept / Approach: ICMP operates alongside IP and is used by routers and hosts to send error and informational messages (for example, Destination Unreachable, Time Exceeded). A “destination network unknown” is conveyed via an ICMP Destination Unreachable message with an appropriate code. It is not a TCP or ARP function, nor a BootP/DHCP bootstrapping mechanism. Step-by-Step Solution: Identify layer: control messaging at the network layer → ICMP.Match error type: Destination Unreachable (network unreachable code).Select ICMP as the protocol delivering the message back to the source. Verification / Alternative check: Packet captures during failed routes show ICMP Destination Unreachable originating from an intermediate router and addressed to the sender’s IP, confirming ICMP’s role. Why Other Options Are Wrong: TCP is a transport protocol for reliable streams; it does not carry network-layer errors. ARP resolves IPv4 addresses to MAC addresses on a local LAN only. BootP is for bootstrapping IP configuration, not error reporting. Common Pitfalls: Blocking ICMP entirely at firewalls, which breaks PMTUD and complicates diagnostics; confusing “ping” (ICMP Echo) with all ICMP functionality. Final Answer: ICMP

Question 3

Meaning of the passive interface command: In common dynamic routing protocols on routers, what does marking an interface as passive actually do?

Accepted Answer

Introduction / Context: Most routing daemons support a “passive” setting to control how a specific interface participates in a routing protocol. This is useful on user-facing LANs where you want to advertise the connected network into the protocol but do not want to send hello or periodic update packets that could form neighbor adjacencies with end hosts. Given Data / Assumptions: We are considering distance-vector protocols (for example, RIP, EIGRP in classic update mode) and the general notion across vendors. Goal: advertise the network without active neighbor formation on that interface. Protocol specifics (like OSPF behavior) may vary, but the exam-style definition focuses on suppressing outbound updates. Concept / Approach: Passive on an interface suppresses sending periodic routing updates/hellos out that interface. The router may still receive and process incoming updates (where applicable), and it continues to advertise the connected network via other active interfaces. This prevents unwanted neighbor relationships while keeping route advertisement in the topology database via other links. Step-by-Step Solution: Enable passive on the user-edge interface.Observe that no protocol hello/updates are transmitted on that interface.Verify that routes learned on other interfaces are still processed and the connected network is still injected into the routing process. Verification / Alternative check: Show commands (for example, show ip protocols / show run) confirm passive status; packet captures reveal absence of outbound protocol packets on the passive interface. Why Other Options Are Wrong: Stopping both send and receive (A) overstates the effect. Blocking all updates to the router (C/D) applies globally, not per interface, and is not the meaning of the passive flag. Common Pitfalls: Assuming passive prevents route learning entirely; confusing protocol nuances (OSPF “passive” prevents adjacency, which effectively stops both directions of hello, but the test definition emphasizes suppression of transmitted updates). Final Answer: Stops an interface from sending periodic dynamic updates but not from receiving updates.

Question 4

DHCP and transport layer choice: At the Transport layer, which protocol does DHCP rely on for message delivery between clients and servers/relays?

Accepted Answer

Introduction / Context: Dynamic Host Configuration Protocol (DHCP) automatically assigns IP addresses and other configuration parameters to clients. Because clients may not yet have an IP or full stack ready, the underlying transport must be simple, connectionless, and broadcast-friendly on local segments. Given Data / Assumptions: DHCP must work before an address is assigned. The protocol should tolerate broadcast and relay via DHCP helpers. We are identifying the Transport layer protocol only. Concept / Approach: DHCP uses UDP: clients send from port 68 to server port 67, initially via broadcast (0.0.0.0 to 255.255.255.255) or via a relay agent (DHCP helper) that forwards the request as unicast to the server. TCP would require a prior connection, which is not possible without established addressing. ARP is a link-layer resolution protocol and IP is the network layer, not transport. Step-by-Step Solution: Observe initial DHCPDISCOVER packets: UDP src port 68 → dst port 67.Note broadcast behavior and relay agent forwarding.Conclude UDP is the Transport layer choice for DHCP. Verification / Alternative check: Packet captures show BOOTP/DHCP over UDP with the specified ports; RFCs for DHCP/BOOTP affirm UDP usage. Why Other Options Are Wrong: TCP requires a handshake and is not used by DHCP. ARP is not a Transport protocol. “IP” is a layer below transport and does not fulfill the transport role. Common Pitfalls: Blocking broadcast UDP 67/68 at access layers; misconfiguring DHCP relay on routers, causing clients to time out. Final Answer: UDP

Question 5

Viewing access-list contents: On a router, which command displays the full contents of all configured access lists so you can review entries and sequence numbers?

Accepted Answer

Introduction / Context: Troubleshooting packet filters requires verifying what is actually configured. Network operating systems provide show commands to list Access Control Entries (ACEs) within access lists, including standard, extended, and named variants. Selecting the correct command prevents guesswork and shortens outage windows. Given Data / Assumptions: We need a command to print all ACLs and their contents. Platform/terminology is aligned with common IOS-style syntax. We are not just checking whether an ACL is applied to an interface. Concept / Approach: The canonical command to view every configured access list and its ACEs is show access-lists (or the more specific show ip access-lists for IP ACLs). By contrast, interface-oriented show commands reveal application status but not the full ACE details of all ACLs. Step-by-Step Solution: Run show access-lists → displays all ACL names/numbers and entries.Optionally use show ip access-lists for IP-specific lists.Correlate with interface attachment using show ip interface if needed. Verification / Alternative check: Executing the command returns ordered ACEs; counters may show hit counts, aiding rule analysis and cleanup. Why Other Options Are Wrong: show all access-lists (A) is not valid syntax. show ip interface (C) and show interface (D) focus on interface status; they do not dump all ACL contents. Common Pitfalls: Confusing “applied where” versus “what is inside”; forgetting that sequence numbers matter when inserting or deleting ACEs. Final Answer: show access-lists

Question 6

Checking if an ACL is active on an interface: Which router command tells you whether an IP access list is applied inbound or outbound on a specific interface, along with other per-interface IP settings?

Accepted Answer

Introduction / Context: After creating an access list, you must apply it inbound or outbound on an interface to have any effect. Verifying attachment requires an interface-centric command, not just a global view of ACL contents. Knowing the correct command speeds up troubleshooting connectivity and security policy enforcement. Given Data / Assumptions: The goal is to discover whether a particular interface has an inbound or outbound ACL applied. The platform uses IOS-style show commands. We also want to see other IP-related interface status (for example, helper addresses, proxy ARP). Concept / Approach: show ip interface displays per-interface IP details, including lines such as “Inbound access list is ” and “Outbound access list is .” This directly answers whether an ACL is enabled on that interface and in which direction. Global ACL listings do not indicate where they are applied. Step-by-Step Solution: Execute show ip interface (or without an argument to list all).Scan the output for inbound/outbound access list assignments.Correlate with show access-lists to review the ACEs if needed. Verification / Alternative check: Attempting to remove the ACL from the interface and repeating the command shows the fields change to “not set,” confirming the linkage. Why Other Options Are Wrong: show access-lists lists contents but not where applied. show interface shows operational stats (line protocol, errors) but not IP ACL bindings. show interface access-lists is not a standard IOS command. Common Pitfalls: Forgetting directionality (inbound vs outbound); assuming an ACL exists implies it is applied; overlooking that ACLs are evaluated top-down with an implicit deny at the end. Final Answer: show ip interface

Question 7

Switching and Broadcast Domains — 12-Port Unmanaged Switch In computer networking fundamentals, if you segment a small local area network (LAN) using a single unmanaged Layer 2 Ethernet switch that has 12 physical ports and you do not configure any …

Accepted Answer

Introduction / Context: Understanding broadcast domains is essential for designing scalable and efficient networks. A broadcast domain is the set of devices that receive a Layer 2 broadcast frame (destination MAC FF:FF:FF:FF:FF:FF). This question checks whether you can differentiate the behavior of hubs, switches, routers, and VLANs with respect to broadcast containment. Given Data / Assumptions: A single Layer 2 Ethernet switch with 12 ports is used. No VLANs are configured; the switch is operating in a default, single-VLAN state. No routers or Layer 3 interfaces are present to segment broadcasts. Concept / Approach: Switches break up collision domains per port (full-duplex eliminates collisions), but they do not break up broadcast domains by default. Only routers (or Layer 3 switches performing inter-VLAN routing) and VLAN boundaries segment broadcast domains. Therefore, an unmanaged or default-configured Layer 2 switch, regardless of port count, creates one broadcast domain. The number of ports affects collision domains, not broadcast domains, unless VLANs are configured. Step-by-Step Solution: Identify device type: Layer 2 switch, default configuration.Recall rule: Switch = many collision domains (per port) but a single broadcast domain unless VLANs are used.Note that routers or Layer 3 interfaces are required to segment broadcasts.Conclude that the entire switch is one broadcast domain in the default state. Verification / Alternative check: Create two VLANs (for example, VLAN 10 and VLAN 20) and assign ports to them; you will now have two broadcast domains. With no VLANs, only one exists across all 12 ports. Why Other Options Are Wrong: 2, 5, 12: These imply VLAN or Layer 3 segmentation, which is not present. 0: Impossible; any Layer 2 segment forms at least one broadcast domain. Common Pitfalls: Confusing collision domains (per switch port) with broadcast domains (per VLAN or Layer 3 boundary). Also assuming the number of ports equals the number of broadcast domains, which is incorrect without VLANs. Final Answer: 1

Question 8

Cisco IOS Maintenance — Upgrading Router IOS from a TFTP Server On a Cisco router, which command copies a new IOS image from a TFTP server into the device's flash memory to perform an upgrade?

Accepted Answer

Introduction / Context: Keeping Cisco IOS up to date is a core administrative task. Administrators commonly stage a new image on a TFTP server and then transfer it to the router's flash memory. Knowing the exact copy syntax prevents outages and failed boots. Given Data / Assumptions: The new IOS image file resides on a reachable TFTP server. The router has IP reachability to the TFTP host (correct interface, default gateway, and ACLs). Sufficient flash space exists to store the image. Concept / Approach: The IOS copy command follows the format copy where common sources/destinations include tftp, flash, running-config, startup-config. To upgrade IOS, you must copy from the TFTP source to the router's flash storage: copy tftp flash. After the transfer, update the boot variable and save configuration if needed, then reload to boot the new image. Step-by-Step Solution: Ensure IP connectivity to the TFTP server (ping).Execute: copy tftp flashEnter TFTP server IP, then the image filename when prompted.Verify with: show flash and show bootvar; set boot system if necessary. Verification / Alternative check: Use dir flash: or verify /md5 flash: to confirm a successful copy and integrity prior to reload. Why Other Options Are Wrong: copy tftp run/start: Copies text configs into running or startup configuration, not IOS images. config net: Legacy configuration retrieval, not image management. copy running-config flash: Backs up config to flash, not an image upgrade. Common Pitfalls: Forgetting to set the correct boot system statement or insufficient flash space, causing the router to boot an old image or fail to boot. Final Answer: copy tftp flash

Question 9

Cisco NVRAM Cleanup — Deleting the Saved Startup Configuration If you need to remove the configuration stored in NVRAM (startup-config) on a Cisco router to return it to default settings, which command would you use?

Accepted Answer

Introduction / Context: Clearing the saved configuration (startup-config) is often necessary before repurposing a device or troubleshooting. On classic IOS, several syntaxes exist; exam-style questions commonly reference the shorthand form used to erase NVRAM content that is loaded at the next boot. Given Data / Assumptions: The goal is to delete the contents of NVRAM (startup-config), not the current running-config in RAM. The device is a Cisco router using traditional IOS syntax. We want the canonical command recognized in entry-level administration tasks. Concept / Approach: The saved configuration that persists across reloads is the startup-config stored in NVRAM (or its flash-backed equivalent). To remove it, classic IOS accepts commands such as erase startup or the more explicit erase startup-config, as well as the legacy write erase. After erasing, a reload boots the device without a saved configuration and triggers the initial configuration dialog. Step-by-Step Solution: From privileged EXEC mode, issue: erase startupConfirm the prompt to erase NVRAM contents.Reload the device: reloadUpon boot, skip initial config or reconfigure as needed. Verification / Alternative check: Run show startup-config after erasure (or before reload) to verify it is empty or not present. After reload, show running-config shows default minimal settings. Why Other Options Are Wrong: erase nvram/delete nvram: Not standard modern IOS commands; syntax varies and may not be valid. erase running: A running-config is in RAM and is cleared by reload, not by this command. erase startup-config: Valid on many IOS versions; however, the keyed option provided as the exam's expected shorthand here is erase startup. Common Pitfalls: Erasing the wrong configuration (confusing running vs. startup) or forgetting to reload, leaving the old running-config active until reboot. Final Answer: erase startup

Question 10

Console Line Configuration — Entering line cons 0 After entering global configuration mode on routerA and typing: routerA(config)# line cons 0 what task can you perform next in this console line context?

Accepted Answer

Introduction / Context: IOS separates configuration contexts to avoid accidental changes. The line configuration mode is used to manage access parameters for console, auxiliary, and vty (Telnet/SSH) lines. Knowing which settings belong to each context improves both security and usability. Given Data / Assumptions: The command entered is line cons 0 from global configuration mode. This selects the console line (physical console) configuration context. The goal is to identify actions appropriate to the console line. Concept / Approach: Within line console 0, administrators can set the console password, configure login behavior, set exec-timeout, enable/disable logging synchronous, and control access parameters for the local console. Telnet and SSH settings belong to line vty contexts, while privileged password (enable secret) is configured in global configuration mode, not within a line context. Step-by-Step Solution: Enter: routerA(config)# line cons 0Set password: password Require login: loginOptionally tune: exec-timeout, logging synchronous, privilege level, etc. Verification / Alternative check: Exit to privileged EXEC and test by reconnecting to the console; you should be prompted for the configured console password if login is enabled. Why Other Options Are Wrong: Set Telnet password: Performed under line vty 0 4, not console. Shut down the router: That is a system action, not a line configuration. Disable console connections: You can restrict or apply transport input none on vty; console physical access cannot be disabled in this way. Configure enable secret: Done in global config, not under line console. Common Pitfalls: Forgetting to add login after setting a password, which leaves the password unused for access control. Final Answer: Set your console password.

Question 11

Backing Up Cisco IOS — Copying the Image from Flash to a TFTP Server Which command backs up the currently running IOS image by copying it from router flash storage to a TFTP server?

Accepted Answer

Introduction / Context: Backing up a known-good IOS image is a best practice before upgrades or hardware changes. The transfer typically uses TFTP due to its simplicity and wide support in network environments. Given Data / Assumptions: The desired source is the device's flash memory where the IOS image resides. The destination is a reachable TFTP server. Network connectivity and adequate permissions exist. Concept / Approach: The IOS copy syntax is copy . For a backup, the source is flash and destination is tftp, giving copy flash tftp. You will be prompted for the TFTP server IP and the filename to write on the server. Step-by-Step Solution: Verify image name: show flashPing the TFTP server to confirm reachability.Run: copy flash tftpProvide server IP and destination filename when prompted. Verification / Alternative check: After the transfer, verify the file on the TFTP server and optionally compare checksums using verify /md5 (on platforms that support it) to ensure integrity. Why Other Options Are Wrong: backup IOS disk: Not a valid IOS command. copy ios tftp: Incorrect object name; source is flash, not "ios". copy tftp flash: This uploads an image to the router (upgrade), not a backup. copy running-config tftp: Backs up configuration, not the IOS image. Common Pitfalls: Confusing configuration backups with image backups and overwriting existing files on the TFTP server unintentionally. Final Answer: copy flash tftp

Question 12

Wi-Fi Standards — 54 Mb/s at 2.4 GHz Which IEEE 802.11 WLAN amendment supports a theoretical maximum data rate of up to 54 Mb/s in the 2.4 GHz band?

Accepted Answer

Introduction / Context: Wireless LAN standards differ by frequency bands and achievable data rates. Memorizing which amendment offers which speeds and bands is a frequent certification requirement and practical for troubleshooting Wi-Fi performance. Given Data / Assumptions: We are comparing 802.11a/b/g/n families at a high level. The target is 54 Mb/s at 2.4 GHz. The question focuses on theoretical maximums, not real-world throughput. Concept / Approach: 802.11g operates in 2.4 GHz and supports up to 54 Mb/s using OFDM, backward compatible with 802.11b. By contrast, 802.11a runs at 5 GHz with up to 54 Mb/s; 802.11b provides 11 Mb/s at 2.4 GHz; 802.11n can use 2.4 or 5 GHz with higher rates via MIMO; 802.11ac (not in the original options list) is 5 GHz only with much higher rates. Step-by-Step Solution: Match frequency: 2.4 GHz narrows choices to b/g/n.Match speed: 54 Mb/s aligns with g (and a at 5 GHz).Conclude 802.11g is the correct amendment for 54 Mb/s at 2.4 GHz.Select option C. Verification / Alternative check: Vendor datasheets and certification study guides consistently list 802.11g as 2.4 GHz with up to 54 Mb/s nominal data rate. Why Other Options Are Wrong: A: 54 Mb/s but at 5 GHz, not 2.4 GHz. B: 11 Mb/s at 2.4 GHz. N: Higher rates, MIMO, 2.4/5 GHz; not limited to 54 Mb/s. AC: 5 GHz only, far higher speeds. Common Pitfalls: Confusing 802.11a and 802.11g because both list 54 Mb/s; the band differentiates them. Final Answer: G

Question 13

NAT Overload — One Public IP for Many Internal Users Which flavor of Network Address Translation enables many inside hosts to share a single public IPv4 address when accessing the Internet by multiplexing with transport ports?

Accepted Answer

Introduction / Context: IPv4 scarcity led to widespread use of address translation at network edges. When many users must share one public address, Port Address Translation (PAT) is typically used to distinguish sessions by port numbers while rewriting source addresses. Given Data / Assumptions: Multiple inside clients access external services simultaneously. The edge device has only one public IPv4 address available. Standard consumer and enterprise NAT behavior is assumed. Concept / Approach: PAT, often called NAT overload, maps many inside local addresses to a single inside global address by translating source transport ports (TCP/UDP) uniquely per flow. Static NAT maps one-to-one between a private and a public address; Dynamic NAT maps from a pool on a one-to-one basis per session but still consumes one public per inside host at a time. Only PAT allows many-to-one at the same time without exhausting the public address pool. Step-by-Step Solution: Identify the requirement: many hosts, single public IP.Eliminate static and dynamic one-to-one mappings.Choose PAT (overload) to multiplex flows by ports.Confirm with typical configuration: ip nat inside source list ACL interface overload. Verification / Alternative check: Observe NAT translation tables: many inside sources map to one global address with distinct port tuples under PAT. Why Other Options Are Wrong: Static/Dynamic: Consume a public IP per inside host, not suitable for many-to-one sharing. NAT (generic): Too broad; does not specify the many-to-one port multiplexing. Double NAT: Refers to two separate NAT domains, not to many-to-one operation. Common Pitfalls: Confusing “dynamic NAT” with PAT; only PAT provides many-to-one simultaneous connections using port translation. Final Answer: PAT

Networking Basics Questions