Question 1

Half-duplex vs full-duplex Ethernet Which statements are unique to half-duplex Ethernet when compared with full-duplex Ethernet?

Accepted Answer

Introduction / Context: Ethernet links can operate in half-duplex or full-duplex. Understanding the operational differences explains why modern networks avoid hubs and prefer switches. Half-duplex inherited the original CSMA/CD model with collision handling, while full-duplex eliminates collisions and increases efficiency. Given Data / Assumptions: Statements provided: (1) operates in a shared collision domain, (2) operates in a private collision domain, (3) has higher effective throughput, (4) has lower effective throughput. We compare half-duplex uniquely against full-duplex. Concept / Approach: Half-duplex means a device cannot transmit and receive simultaneously and must contend for the medium, creating a shared collision domain. Consequently, bandwidth is shared and effective throughput is lower than the nominal line rate. Full-duplex dedicates a switch port to a single device, creating a private, collision-free environment with higher throughput and no CSMA/CD. Step-by-Step Solution: Map statement (1) to half-duplex: true (shared collision domain).Statement (2) describes full-duplex: false for half-duplex.Statement (3) claims higher throughput for half-duplex: false.Statement (4) lower throughput: true for half-duplex.Therefore, statements (1) and (4) are correct. Verification / Alternative check: Throughput tests with identical hardware show fewer retransmissions and higher goodput in full-duplex due to elimination of collisions and backoff timers, confirming that half-duplex yields lower effective throughput. Why Other Options Are Wrong: (2 only) misattributes full-duplex characteristics to half-duplex. (1,2 and 3) includes contradictory and incorrect claims. (4 only) ignores the shared collision domain property. (1 and 3) incorrectly asserts higher throughput for half-duplex. Common Pitfalls: Mismatched duplex settings between NIC and switch cause late collisions and poor performance. Assuming auto-negotiation always selects optimal duplex; verify with show interfaces. Final Answer: 1 and 4

Question 2

Common flow control mechanisms Which of the following are considered flow control techniques in networking?

Accepted Answer

Introduction / Context: Flow control regulates the rate of data transmission between sender and receiver to prevent buffer overflows and ensure reliable delivery. It operates at multiple layers, from link-layer buffering to transport-layer schemes like sliding windows, and network-level strategies that avoid sustained congestion. Given Data / Assumptions: (1) Buffering, (2) Cut-through, (3) Windowing, (4) Congestion avoidance. Goal: identify techniques that actively manage data flow or backpressure. Concept / Approach: Buffering absorbs bursts and smooths traffic at ingress/egress queues. Windowing (for example, TCP sliding window) limits the amount of unacknowledged data in flight, dynamically adjusting to receiver capacity. Congestion avoidance (such as RED/WRED, TCP congestion window adjustments) proactively reduces offered load before buffers overflow. Cut-through is a switching method that reduces latency by forwarding frames before full reception; it is not a flow control mechanism. Step-by-Step Solution: Select buffering: yes, prevents receiver overrun.Evaluate cut-through: forwarding technique, not flow control → exclude.Select windowing: yes, classic transport-layer flow control.Select congestion avoidance: yes, controls sending behavior to avert drops. Verification / Alternative check: Observe TCP throughput under packet loss: window reductions and slow-start demonstrate flow control plus congestion control working together to stabilize traffic. Why Other Options Are Wrong: Options A and C include cut-through, which does not regulate flow. Option D ignores buffering and congestion avoidance, both recognized techniques. Option E omits windowing, a core mechanism in TCP/IP. Common Pitfalls: Confusing low-latency switching (cut-through) with rate regulation. Assuming congestion avoidance is only QoS; it is integral to flow stability. Final Answer: 1, 3 and 4 (Buffering; Windowing; Congestion avoidance)

Question 3

Full-duplex Ethernet behavior Which three statements are true about the operation of a full-duplex Ethernet network?

Accepted Answer

Introduction / Context: Full-duplex Ethernet allows simultaneous send and receive on twisted-pair or fiber links, eliminating collisions and the need for CSMA/CD. Knowing the operational requirements and consequences of full-duplex is essential for designing modern switched networks with predictable performance. Given Data / Assumptions: (1) No collisions in full-duplex mode. (2) A dedicated switch port is required for each full-duplex node. (3) Ethernet hub ports are preconfigured for full-duplex mode. (4) NIC must check media availability before transmitting in full-duplex. (5) Both the NIC and switch port must support full-duplex. Concept / Approach: In full-duplex Ethernet, there is no shared medium and no contention; therefore collisions do not occur. Each device uses a dedicated switch port; hubs operate only in half-duplex and cannot be full-duplex. CSMA/CD (carrier sense, collision detect) is disabled in full-duplex, so a NIC does not perform carrier checks before sending. Both link partners must advertise or be configured for full-duplex for it to function properly. Step-by-Step Solution: Validate (1): True—no collisions in full-duplex.Validate (2): True—requires a dedicated switch port.Validate (3): False—hubs are half-duplex devices.Validate (4): False—CSMA/CD disabled; no need to check carrier in full-duplex.Validate (5): True—both ends must support full-duplex. Verification / Alternative check: On IOS, show interfaces confirms duplex; counters show zero collisions on full-duplex links under normal operation. Why Other Options Are Wrong: Combinations including (3) or (4) are invalid due to hub limitations and the absence of CSMA/CD in full-duplex. “5 only” omits other true characteristics. Common Pitfalls: Mismatched duplex settings (auto vs hard-set) lead to late collisions and poor throughput. Assuming a hub can operate full-duplex—this is not possible. Final Answer: 1, 2, and 5

Question 4

Why segment with a bridge? What are two primary purposes for network segmentation using a bridge (or a modern Layer 2 switch)?

Accepted Answer

Introduction / Context: Bridges and modern Layer 2 switches segment LANs into smaller collision domains. By forwarding frames based on MAC addresses and learning, they reduce collisions and increase effective bandwidth. However, they do not break broadcast domains unless VLANs or Layer 3 boundaries are introduced. Given Data / Assumptions: (1) Add more broadcast domains. (2) Create more collision domains. (3) Add more bandwidth for users. (4) Allow more broadcasts for users. Concept / Approach: Each switch port is its own collision domain, eliminating collisions seen with hubs. This segmentation means fewer devices contend for the same medium, raising per-user throughput—effectively more bandwidth. Broadcast domains are not changed by simple bridging; broadcasts still flood all ports in the same VLAN or bridge group. Step-by-Step Solution: Confirm (2): Yes, more (smaller) collision domains per port.Confirm (3): Yes, less contention yields higher effective user bandwidth.Evaluate (1): No, broadcast domains remain the same without VLANs or Layer 3 routing.Evaluate (4): No, bridges do not intentionally increase broadcast traffic. Verification / Alternative check: Measuring throughput before and after replacing a hub with a switch shows reduced collisions and higher goodput; broadcast volume remains similar unless VLANs are configured. Why Other Options Are Wrong: (1 only) and (4 only) misunderstand broadcast-domain behavior. (2 and 4) claims more broadcasts, which is not an intended outcome. (1 and 2) mixes a false claim with a true one. Common Pitfalls: Assuming switches reduce broadcasts by default; VLANs or routers are needed to contain broadcasts. Overlooking duplex mismatches when migrating from hubs. Final Answer: 2 and 3 (Create more collision domains; Add more bandwidth for users)

Question 5

OSI layer mapping for common devices and apps Fill in the layers: Routers operate at layer ___; LAN switches operate at layer ___; Ethernet hubs operate at layer ___; word processing applications operate at layer ___.

Accepted Answer

Introduction / Context: The OSI reference model provides a layered way to understand networking functions. Correctly mapping common devices to their primary layers helps in troubleshooting and design. While devices can process information at multiple layers, each has a primary role associated with a specific OSI layer. Given Data / Assumptions: Devices: router, LAN switch, Ethernet hub, and an end-user word processing application. We are mapping each to its principal OSI layer of operation. Concept / Approach: Routers make forwarding decisions using Layer 3 (Network) information such as IP addresses. LAN switches forward based on Layer 2 (Data Link) MAC addresses and maintain MAC tables per VLAN. Hubs simply repeat bits at Layer 1 (Physical), creating a single shared collision domain. Applications like word processors operate at Layer 7 (Application), using network services provided by lower layers. Step-by-Step Solution: Router → Layer 3 (routing tables, IP subnets).LAN switch → Layer 2 (MAC learning, frame switching).Ethernet hub → Layer 1 (bit-level repeat, no filtering).Word processor → Layer 7 (user-facing application). Verification / Alternative check: Use CLI outputs: a switch show mac address-table confirms L2 functions; a router show ip route highlights L3. A hub lacks such intelligence and merely regenerates signals. Why Other Options Are Wrong: Options placing switches at Layer 3 conflate multilayer switching with basic LAN switching. “None” for applications ignores the Application layer role. Any mapping of hubs beyond Layer 1 is incorrect; hubs do no frame or packet processing. Common Pitfalls: Confusing multilayer (L3) switches with pure L2 operation; here the question specifies “LAN switches,” implying L2. Assuming apps operate at multiple layers; while they rely on lower layers, their functional layer is L7. Final Answer: 3, 2, 1, 7

Question 6

Core router capabilities Which of the following describe functions that routers commonly perform in an IP internetwork?

Accepted Answer

Introduction / Context: Routers interconnect multiple IP networks and forward packets between them. Understanding their key functions clarifies how traffic is directed, controlled, and secured in enterprise and service-provider environments. While some tasks overlap with other devices, routers remain central to inter-network operations. Given Data / Assumptions: The functions listed are packet switching, packet filtering, internetwork communication, and path selection. We consider typical router capabilities with modern IOS or equivalent software. Concept / Approach: Packet switching is the fundamental act of forwarding packets from an ingress interface to an egress interface based on a routing table (or FIB/CEF). Packet filtering is achieved through ACLs, firewall features, or zone-based policies that permit or deny traffic. Internetwork communication is the core purpose: connecting different IP networks and enabling hosts on separate subnets to exchange data. Path selection occurs via routing protocols and administrative policies that determine the best next hop to reach a destination network. Step-by-Step Solution: Confirm that a router learns routes (static/dynamic) and installs best paths in the RIB/FIB.Verify packet forwarding using show ip cef or show ip route plus interface counters.Apply ACLs to demonstrate packet filtering and observe counters increment.Validate inter-VLAN or inter-subnet communication through successful end-to-end pings across routed boundaries. Verification / Alternative check: Traceroute reveals path selection decisions; ACL logs or counters prove filtering; throughput tests confirm switching performance. Combined, these demonstrate all listed functions. Why Other Options Are Wrong: Choosing any single subset ignores the breadth of router capabilities. Routers routinely perform all four listed functions in production networks. Common Pitfalls: Confusing switching (Layer 2) with packet switching (Layer 3 forwarding); routers perform the latter. Assuming filtering is only a firewall task; routers implement ACL-based filtering extensively. Final Answer: All of the above

Question 7

TCP reliability and receiver flow control: A receiving host has detected that some expected TCP segments are missing (not all segments have arrived to be acknowledged). To improve the reliability and stability of this communication session without t…

Accepted Answer

Introduction / Context: In Transmission Control Protocol (TCP), reliable delivery depends on acknowledgments, retransmissions, and flow control. If a receiver notices that not all segments are arriving (gaps in sequence numbers), it can signal the sender to slow down so buffers do not overflow and retransmissions can catch up. Given Data / Assumptions: Some TCP segments have not been received by the destination host. The session should continue without being torn down. Standard TCP mechanisms such as ACKs, sliding windows, and retransmissions are in use. Concept / Approach: TCP uses a sliding window for flow control. The receiver advertises a window size (rwnd) in ACKs. By reducing the advertised window, the receiver throttles the sender's transmit rate, preventing buffer overflow and allowing recovery via retransmissions and selective acknowledgments (if supported). Step-by-Step Solution: Receiver observes missing segments (out-of-order or gaps in sequence numbers).Receiver sends ACKs with a reduced receive window value.Sender, honoring the smaller window, sends less data in flight.Retransmission (triggered by duplicate ACKs, timeout, or SACK) fills the gap; session stabilizes. Verification / Alternative check: Packet capture should show ACKs advertising smaller window sizes and subsequent sender behavior limiting outstanding data. Throughput will decrease while reliability improves. Why Other Options Are Wrong: Sending a different source port (A) would create a new session, not fix flow control. Restarting the virtual circuit (B) is disruptive and unnecessary for simple loss. Decreasing the sequence number (C) is invalid; sequence numbers are dictated by bytes sent, not manually adjusted midstream. Common Pitfalls: Confusing congestion control (sender-side cwnd) with receiver flow control (rwnd); attempting to reset sessions unnecessarily; ignoring that a too-small window can overly throttle good links. Final Answer: Decrease the window size.

Question 8

Layer 1 expansion of a single LAN segment: Which physical layer (Layer 1) devices can be used to enlarge the area covered by a single Ethernet LAN segment?

Accepted Answer

Introduction / Context: Ethernet collision domains in classic half-duplex environments can be extended at Layer 1 using devices that simply regenerate and repeat signals without interpreting frames. Understanding which components truly operate at Layer 1 helps avoid mixing in Layer 2 devices that change the broadcast/collision domain model. Given Data / Assumptions: The listed devices are: 1) Switch, 2) NIC, 3) Hub, 4) Repeater, 5) RJ45 transceiver. The goal is to enlarge a single LAN segment, not to create separate segments. Focus is strictly on OSI Layer 1 behavior. Concept / Approach: Layer 1 devices forward bits without MAC learning or filtering. Repeaters (including multiport repeaters, i.e., hubs) extend signal reach and timing but keep a single collision domain. Switches are Layer 2 and segment collision domains; NICs and connectors/transceivers are endpoints/physical media, not segment enlargers by themselves. Step-by-Step Solution: Identify L1 devices: repeater and hub.Confirm they regenerate the signal and extend reach but do not break the collision domain.Conclude valid choices: items 3 and 4. Verification / Alternative check: Topology drawings show a larger shared collision domain when hubs/repeaters connect multiple cables; switches would instead create multiple segments. Why Other Options Are Wrong: Switches (1) are Layer 2; they do not enlarge the same segment—they create new segments. RJ45 transceivers (5) and NICs (2) are not used to extend a segment's geographic scope by repeating signals. Common Pitfalls: Assuming modern switches are Layer 1 because they have many ports; confusing media converters with repeaters; forgetting that hubs create a single collision domain that scales poorly. Final Answer: 3 and 4

Question 9

Encapsulation order across the stack: When data moves down the networking stack for transmission, what is the correct encapsulation order from highest-level data to the actual signals on the medium?

Accepted Answer

Introduction / Context: Encapsulation wraps higher-layer information in lower-layer headers/trailers as data descends the stack. Getting the order right helps troubleshoot where problems occur (e.g., TCP vs IP vs Ethernet) and clarifies what each layer contributes. Given Data / Assumptions: Application payload is called “data.” Transport adds a header to create a segment (TCP) or datagram (UDP). Network adds IP to create a packet. Data link (e.g., Ethernet) adds a frame header/trailer. Physical transmits bits on the medium. Concept / Approach: The canonical progression is: Data (Layer 7–5 payload) → Segment (Layer 4) → Packet (Layer 3) → Frame (Layer 2) → Bits (Layer 1). Each layer adds control information relevant to its function (reliability, addressing, media access). Step-by-Step Solution: Start with application data.Transport encapsulation → segment.Network encapsulation → packet (IP header).Data link encapsulation → frame (MAC addresses, FCS).Physical layer transmits bits. Verification / Alternative check: Packet captures show nested headers: Ethernet outside, then IP, then TCP/UDP, then application data, matching the sequence above. Why Other Options Are Wrong: Options that swap frame/packet or segment/data contradict OSI/TCP-IP encapsulation rules. “Segment, data…” places payload after transport encapsulation, which is incorrect. Common Pitfalls: Confusing “packet” and “frame” terminology; forgetting that at Layer 4 TCP is a segment while UDP is often called a datagram. Final Answer: Data, segment, packet, frame, bit

Question 10

TCP flow control purpose: In reliable transport protocols such as TCP, what is the primary purpose of flow control between sender and receiver?

Accepted Answer

Introduction / Context: Transport protocols implement mechanisms to avoid overwhelming receivers. TCP uses a sliding window scheme so the receiver can advertise how much data it can accept before acknowledgment, preventing buffer overflow and improving end-to-end reliability. Given Data / Assumptions: TCP provides reliable, ordered byte-stream delivery. Receivers have finite buffer capacity and processing speed. Flow control is separate from error control and congestion control. Concept / Approach: Flow control is receiver-driven: the receiver advertises a window size (rwnd) in ACKs indicating how much unacknowledged data the sender may have in flight. The sender must respect this limit, adjusting its send rate accordingly. Step-by-Step Solution: Receiver determines free buffer space.Receiver sets window field in ACKs to reflect available space.Sender limits outstanding bytes ≤ advertised window.As data is processed, receiver increases the window in subsequent ACKs. Verification / Alternative check: Analyze a trace: the TCP header window field fluctuates with buffer availability; sender pacing changes accordingly. Why Other Options Are Wrong: Retransmission on missing ACKs (A) is error control, not flow control. Reassembly order (B) is provided by sequence numbers and buffering. Segment size regulation (D) is influenced by MSS/MTU, not flow control per se. Common Pitfalls: Confusing congestion control (network capacity) with flow control (receiver capacity); assuming fixed window sizes when modern TCP can use window scaling. Final Answer: To provide a means for the receiver to govern the amount of data sent by the sender.

Question 11

Choosing cabling resistant to electromagnetic interference (EMI): You must implement a network medium that is essentially immune to EMI and radio-frequency interference. Which cabling type should you use?

Accepted Answer

Introduction / Context: Electromagnetic interference can corrupt signals carried over copper media. In environments with heavy machinery, medical equipment, or high EMI/RFI, choosing the right medium is critical for reliable networking. Given Data / Assumptions: High-EMI environment or need for maximum immunity to interference. Options include copper and optical media. Goal is to avoid susceptibility to external electromagnetic fields. Concept / Approach: Fiber-optic cables transmit light, not electrical signals. They are immune to EMI/RFI, offer excellent security against tapping, and support long distances and high bandwidth. Copper (UTP/coax) conducts electricity and can both emit and receive interference unless heavily shielded; even then, immunity is not comparable to fiber. Step-by-Step Solution: Identify EMI risk → prefer non-electrical medium.Compare options: coax and UTP are copper; fiber uses light.Select fiber-optic as the optimal EMI-immune choice. Verification / Alternative check: Vendor datasheets and standards note fiber’s inherent EMI immunity and typical distances (hundreds of meters to kilometers depending on type), versus copper's shorter reach and susceptibility. Why Other Options Are Wrong: Thicknet/Thinnet coax (A/B) and Cat5 UTP (C) are copper media and can pick up noise; shielding can help but not eliminate susceptibility. Common Pitfalls: Assuming shielded twisted pair (STP) equals immunity; overlooking grounding issues that can introduce noise on copper runs. Final Answer: Fiber-optic cable

Question 12

Console connection cabling: Which RJ-45 UTP cable type is normally used to connect a PC’s serial COM port (via an RJ-45 console adapter) to a router or switch console port for out-of-band management?

Accepted Answer

Introduction / Context: Out-of-band console access to Cisco devices typically uses an RJ-45 console port and a special pinout cable. Choosing the correct cable avoids frustrating connection failures when performing initial configuration or recovery. Given Data / Assumptions: PC serial COM port is adapted to RJ-45 for console access. Device offers an RJ-45 console port. Standard Cisco cabling conventions apply. Concept / Approach: A rollover (rolled) cable reverses pin 1 to pin 8, 2 to 7, etc. It is distinct from a straight-through (patch) or crossover (switch-to-switch) Ethernet cable. For console sessions, the rolled cable plus a terminal emulator (and often a USB-to-serial adapter) provides management access independent of network state. Step-by-Step Solution: Identify console port requirements → serial signaling, not Ethernet data.Select rolled cable (light-blue console cable in Cisco kits).Connect PC → adapter → rolled cable → device console; set terminal parameters (9600-8-N-1 by default). Verification / Alternative check: Successful terminal session shows ROMMON/IOS prompts. Using a straight-through/crossover typically fails to produce output due to wrong pin mapping. Why Other Options Are Wrong: Straight-through and crossover cables are for Ethernet data links, not console serial connections. “Crossover with a CSU/DSU” is unrelated to local console access. Common Pitfalls: Confusing Ethernet link types with console wiring; forgetting that the console is not IP-based and won’t bring up link LEDs like Ethernet ports. Final Answer: Rolled

Question 13

Why the industry uses the OSI reference model: Which statements best explain the advantages of the layered OSI reference model for networking?

Accepted Answer

Introduction / Context: The OSI model provides a conceptual framework for standardizing network functions into layers. This abstraction benefits design, interoperability, and troubleshooting by separating concerns and defining clear responsibilities at each layer. Given Data / Assumptions: Statement 1: Divides communication into simpler components aiding development, design, and troubleshooting. Statement 2: Enables vendors to use the same electronic components (cost saving claim). Statement 3: Supports multiple competing standards to create business opportunities. Statement 4: Encourages standardization by defining functions per layer. Concept / Approach: True benefits focus on modularity (1) and standardization (4). The OSI model does not dictate identical hardware components (2) nor promote competing standards (3); rather, it promotes interoperable standards mapped to layers. Step-by-Step Solution: Evaluate each statement against OSI goals.Accept 1 (modularity/troubleshooting) and 4 (standardized functions).Reject 2 and 3 as misconceptions. Verification / Alternative check: Standards bodies (ISO/IEC, IEEE, IETF) define protocols per layers; vendor interoperability proofs echo advantages in statements 1 and 4. Why Other Options Are Wrong: 2: Same components are not an OSI goal; vendors implement differently. 3: OSI seeks interoperability, not proliferation of incompatible standards. Common Pitfalls: Assuming OSI is an implementation model; conflating OSI with TCP/IP suite specifics. Final Answer: 1 and 4

Question 14

OSI layer for segmentation: At which OSI layer is a continuous data stream segmented into smaller transport units for delivery across the network?

Accepted Answer

Introduction / Context: Large application data streams are broken into manageable pieces for reliable delivery. Identifying the OSI layer responsible for segmentation helps map functions like flow control, sequencing, and reassembly to the proper protocol stack layer. Given Data / Assumptions: Segmentation refers to dividing the byte stream into segments (e.g., TCP). Layer responsibilities follow the OSI model. Reliable, ordered delivery is typically provided at the transport layer. Concept / Approach: The Transport layer (Layer 4) handles segmentation, sequencing, acknowledgment, and reassembly at the destination. TCP is the canonical example. Lower layers forward packets/frames; higher layers provide application semantics. Step-by-Step Solution: Application produces data stream.Transport layer segments and numbers the data.Network layer encapsulates segments into packets and routes them.Data link/Physical deliver frames/bits over the medium. Verification / Alternative check: Packet analyzers show TCP sequence numbers and segment sizes—clear evidence of Layer 4 segmentation and reassembly behavior. Why Other Options Are Wrong: Physical (A) transmits bits; Data Link (B) frames; Network (C) handles logical addressing and routing, not segmentation of application data. Common Pitfalls: Confusing fragmentation (Network layer, IP) with segmentation (Transport layer). Final Answer: Transport

Question 15

Ethernet CSMA/CD behavior after collisions: Which statements correctly describe how hosts determine when to transmit on a CSMA/CD Ethernet LAN, especially after a collision occurs?

Accepted Answer

Introduction / Context: Classic shared-media Ethernet uses Carrier Sense Multiple Access with Collision Detection (CSMA/CD). Understanding the rules for sensing, deferring, and retransmitting after collisions is fundamental to legacy Ethernet operations and exam objectives. Given Data / Assumptions: Multiple hosts share one collision domain (hub or half-duplex link). CSMA/CD dictates when a host can transmit. After collisions, a defined backoff algorithm prevents repeated simultaneous retransmissions. Concept / Approach: Correct statements are: (2) stations must wait until the medium is idle before transmitting, and (5) after a collision, all stations perform a random backoff; when their timers expire, they all have equal priority to transmit. Statement (1) is false because simultaneous successful transmissions cannot occur on the same collision domain. Statements (3) and (4) mischaracterize hubs and priority rules. Step-by-Step Solution: Sense carrier; if busy, defer (rule 2).If a collision occurs, jam signal is sent.All stations invoke exponential random backoff (rule 5).When backoff expires and medium is idle, a station attempts retransmission with no innate priority. Verification / Alternative check: Ethernet specifications define binary exponential backoff; trace captures show jam sequences and randomized retransmission intervals after collisions. Why Other Options Are Wrong: 1: Simultaneous success is impossible on a shared collision domain. 3: Adding hubs increases collisions; it does not improve CSMA/CD efficiency. 4: No host gains first priority after a collision; all defer and randomize. Common Pitfalls: Applying CSMA/CD rules to full-duplex switched Ethernet (no collisions); assuming priority mechanisms exist where they do not. Final Answer: 2 and 5

Question 16

Fields found in an IEEE Ethernet frame header: Which of the following fields are included in the Ethernet frame header (not the payload) as defined by IEEE framing?

Accepted Answer

Introduction / Context: An IEEE 802.3 Ethernet frame consists of a header, payload, and trailer. Knowing which fields live in the header versus payload or trailer is essential for interpreting captures and understanding frame processing by NICs and switches. Given Data / Assumptions: Field 1: Source and destination MAC address. Field 2: Source and destination network address (IP addresses). Field 3: Combination of MAC and network addresses (not a real header field). Field 4: FCS field. Concept / Approach: The Ethernet header includes Destination MAC, Source MAC, and a Length/Type field (and optional 802.1Q tag). IP addresses are part of the Layer 3 packet, not the Layer 2 header. The Frame Check Sequence (FCS) is a trailer used for error detection. Among the provided groupings, the best correct selection that references actual Ethernet control fields and excludes IP addresses is the pairing of MAC addressing (1) and the FCS integrity field (4) found in the frame structure (header+trailer). Step-by-Step Solution: Acknowledge header carries MAC addresses (Layer 2).Reject IP addressing as header content (Layer 3).Recognize FCS exists in the frame (trailer) for error checking.Select the option that reflects real Ethernet control fields while excluding IP addresses. Verification / Alternative check: Refer to Ethernet frame format diagrams: Preamble/SFD (not part of the MAC header proper), Destination MAC, Source MAC, Length/Type, optional tag, payload, and FCS trailer. Why Other Options Are Wrong: “2 only” and “2 and 3 only” include IP addresses, which are not in the Ethernet header. “3 only” is not an actual field definition. Common Pitfalls: Equating frame header with entire frame; confusing Layer 2 MAC addressing with Layer 3 IP addressing; overlooking that FCS is a trailer, not header, though it is a defined field of the frame structure. Final Answer: 1 and 4

Question 17

Number systems in networking — convert binary to decimal and hexadecimal What are the decimal and hexadecimal equivalents of the 8-bit binary value 10110111 (commonly seen in subnet masks, flags, and protocol fields)?

Accepted Answer

Introduction / Context: Networking engineers routinely move between binary, decimal, and hexadecimal when reading packet headers, subnet masks, and hardware addresses. Converting a binary octet such as 10110111 to its decimal and hexadecimal forms strengthens your ability to parse protocol values quickly in tools like Wireshark and router CLIs. Given Data / Assumptions: Binary value: 10110111 (8 bits). We must produce decimal and hexadecimal equivalents. Standard positional notation applies: bit weights 128, 64, 32, 16, 8, 4, 2, 1. Concept / Approach: For decimal, sum the powers of two where the bit is 1. For hexadecimal, group the 8 bits into two 4-bit nibbles, convert each nibble to a hex digit, and combine. Hex shorthand is widely used in documentation (for example, 0xB7). Step-by-Step Solution: Write bit weights: 128 64 32 16 8 4 2 1.Map the bits: 1 0 1 1 0 1 1 1.Add positions with 1s: 128 + 32 + 16 + 4 + 2 + 1.Compute decimal: 128 + 32 = 160; 160 + 16 = 176; 176 + 4 = 180; 180 + 2 = 182; 182 + 1 = 183.Group into nibbles: 1011 0111.Convert each nibble: 1011 = B, 0111 = 7.Hex value: B7 (or 0xB7). Verification / Alternative check: Use a quick mental check: 183 is close to 192 (11000000). Since 10110111 differs by 9 from 11000000, 192 - 9 = 183, consistent with our sum. For hex, confirm that 0xB7 = 11*16 + 7 = 176 + 7 = 183. Why Other Options Are Wrong: A: 69/0x2102 mixes unrelated router register notation and wrong conversion. C: 173/A6 is inconsistent; A6 hex equals 166 decimal, not 173. D: 83/0xC5 mismatches both bases for the given bits. E: 97/0x61 corresponds to 01100001, not 10110111. Common Pitfalls: Forgetting to group nibbles from the left, or misreading bit weights (especially 16 vs. 32). Always double-check with a quick hex-to-decimal recomputation. Final Answer: 183/B7

Question 18

Cabling between Ethernet switches — choose the correct RJ-45 UTP type In a traditional setup without auto-MDI/MDI-X, which twisted-pair cable should be used to directly interconnect two switches at their copper Ethernet ports?

Accepted Answer

Introduction / Context: Correct cabling ensures that transmit (TX) pairs on one device land on receive (RX) pairs on the other. Historically, devices of the same type (switch-to-switch, host-to-host) needed crossover cables; different types (host-to-switch) used straight-through cables. Auto-MDI/MDI-X increasingly removes this requirement, but the underlying rule remains important for exams and legacy gear. Given Data / Assumptions: Two Ethernet switches with RJ-45 copper ports. No auto-MDI/MDI-X or it is disabled. Standard 10/100/1000Base-T pinouts apply. Concept / Approach: Same-type devices transmit and receive on the same pin pairs, so a crossover swaps the relevant pairs (1↔3 and 2↔6 on 10/100Base-T). With auto-MDI/MDI-X, either cable often works, but the canonical answer for two switches is the crossover cable. Step-by-Step Solution: Identify device types: switch-to-switch (same type).Apply cabling rule: same type → crossover, different type → straight-through.Choose crossover as the correct cable. Verification / Alternative check: Check port LEDs and link status after connecting. If auto-MDI/MDI-X is present, straight-through may still link; however, in exam contexts, the expected answer is crossover. Why Other Options Are Wrong: A: Straight-through is for host-to-switch traditionally. C/D: CSU/DSU is for WAN serial links, unrelated to Ethernet copper patching. E: Rolled cable is for console connections to a serial port, not data links. Common Pitfalls: Confusing console rollover with crossover; and assuming auto-MDI/MDI-X applies to all gear—older hardware requires the correct cable type. Final Answer: Crossover cable

Question 19

OSI reference model — which layer provides acknowledgments, sequencing, and flow control? Identify the OSI layer responsible for reliable delivery features such as positive ACKs, sequence numbering, and window-based flow control.

Accepted Answer

Introduction / Context: To troubleshoot end-to-end connectivity, you must map symptoms to the correct OSI layer. Reliability mechanisms—acknowledgments, sequencing, and flow control—determine whether segments arrive intact, in order, and at a manageable rate. These functions are central to TCP and defined at the Transport layer in the OSI model. Given Data / Assumptions: We refer to the classic OSI 7-layer model. Examples include TCP features like ACKs, sequence numbers, and sliding windows. We distinguish per-hop behaviors (Layers 1–3) from end-to-end behaviors (Layer 4 and above). Concept / Approach: Layer 4 (Transport) provides end-to-end communication services for applications. TCP, a Transport protocol, implements reliable transport using acknowledgments, sequence/acknowledgment numbers, and flow control via sliding windows and receive buffers. UDP, also at Layer 4, omits these features to provide low-latency, connectionless delivery. Step-by-Step Solution: Associate ACKs and sequencing with TCP behavior.Map TCP to OSI Layer 4 (Transport).Conclude that the responsible OSI layer is Layer 4. Verification / Alternative check: Packet captures show TCP headers containing sequence and acknowledgment numbers and window sizes—these fields are absent from Layer 3 IP headers. Why Other Options Are Wrong: A: Layer 2 ensures reliable delivery on a local segment (for example, frame checks) but not end-to-end sequencing. B: Layer 3 handles routing and logical addressing (IP), not ACK/sequence/flow control. D: Layer 7 is application logic, relying on services from lower layers. E: Layer 1 is purely physical signaling. Common Pitfalls: Attributing “reliability” generically to Layer 3; while some link technologies retransmit at Layer 2, true end-to-end reliability belongs to Layer 4. Final Answer: Layer 4 (Transport)

Question 20

Full-duplex Ethernet — which link types can operate in full duplex? From the list of common Ethernet connections, select all that can legitimately operate in full-duplex mode (no collisions) when supported by the devices and cabling.

Accepted Answer

Introduction / Context: Full-duplex Ethernet removes collisions by allowing simultaneous send and receive on twisted pairs or fiber. Knowing which device pairs can run full-duplex is essential for configuring links correctly and avoiding mismatch errors that degrade performance. Given Data / Assumptions: Choices: (1) Hub to hub, (2) Switch to switch, (3) Host to host, (4) Switch to hub, (5) Switch to host. Devices support full-duplex and autosensing where applicable. Hubs are half-duplex by design (shared collision domain). Concept / Approach: Full-duplex requires a point-to-point link with no shared collision domain. Switch ports are full-duplex capable; hubs are not. Two modern NICs directly connected (with appropriate cabling or auto-MDI/MDI-X) can negotiate full-duplex as well. Therefore, valid full-duplex pairs are switch-to-switch, switch-to-host, and host-to-host. Hub-involved links remain half-duplex. Step-by-Step Solution: Eliminate any option containing a hub as an endpoint for full-duplex (1 and 4 are invalid).Confirm switch-to-switch is full-duplex capable (2).Confirm switch-to-host is full-duplex capable (5).Confirm host-to-host can run full-duplex with proper cabling (3).Combine valid pairs: 2, 3, and 5. Verification / Alternative check: On Cisco gear, show interfaces displays duplex settings. Two hosts linked directly (for example, lab PCs) will negotiate full-duplex if NICs and drivers support it. Why Other Options Are Wrong: A/B: Include hub links which cannot be full-duplex. E: Omits switch-to-switch, which is full-duplex capable. Common Pitfalls: Duplex mismatches (one end half, the other full) create late collisions and FCS errors. Always verify both ends negotiate or statically match. Final Answer: 2, 3 and 5

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