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Directly connecting two Ethernet hosts Two PCs are cabled back-to-back via their Ethernet NICs using a straight-through patch cord. Pings fail. Which actions will restore connectivity in this simple host-to-host scenario?

Difficulty: Easy

Correct Answer: 1 and 5 only

Explanation:


Introduction / Context:
When two end hosts are connected directly without a hub or switch, the physical and Layer 3 parameters must be appropriate for link negotiation and IP communication. Many classic NICs require a crossover cable for transmit/receive pair alignment. Additionally, both hosts must share a common IP subnet mask so that ARP resolution and local delivery occur without a gateway.


Given Data / Assumptions:

  • Items listed: (1) Use a crossover cable, (2) Use a rollover cable, (3) Set mask 255.255.255.192, (4) Configure a default gateway, (5) Set mask 255.255.255.0.
  • Auto-MDI/MDIX is not assumed (older hardware).
  • The goal is direct L2/L3 communication between two hosts only.


Concept / Approach:

Back-to-back host links traditionally need a crossover cable so that one host’s transmit pair hits the other’s receive pair. A rollover cable is for console connections, not Ethernet data. At Layer 3, hosts must share an IP network; a common /24 like 255.255.255.0 is typical for lab setups. A default gateway is unnecessary for local traffic; ARP resolves peer MACs directly on the same subnet.


Step-by-Step Solution:

Replace straight-through with a crossover cable (item 1) if auto-MDI/MDIX is absent.Configure both hosts with IPs in the same /24 and mask 255.255.255.0 (item 5).Verify link LEDs and test ping again.Confirm ARP tables populate with the peer’s MAC address.


Verification / Alternative check:

Run ipconfig /all (Windows) or ip a (Linux) to verify mask and ARP table entries. Modern NICs with auto-MDI/MDIX may work with straight-through cabling; the core requirement remains that both hosts be in the same IP subnet.


Why Other Options Are Wrong:

  • (2) Rollover is console wiring; it will not pass Ethernet frames.
  • (3) 255.255.255.192 (/26) could work only if both hosts share matching addresses within the same /26; it is not intrinsically required by the scenario. The question pairs it with default gateway (4) which is unnecessary for local pings.
  • (4) Default gateway is only for off-subnet traffic.


Common Pitfalls:

  • Assuming a default gateway is needed for every ping; if peers are local, it is not.
  • Overlooking NIC auto-crossover capabilities on newer equipment.


Final Answer:

1 and 5 only

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