IP Addressing Troubleshooting Capstone

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Lab 10 of 10 in IP Addressing & Subnetting · ← Previous

CCNA exam domain: Network Fundamentals

Advanced IP addressing capstone, run as a fault hunt. A branch build has been handed over and nothing quite works: one LAN cannot reach its gateway, a transit link behaves as though the two ends are on different networks, one router will not accept traffic on a point-to-point link, and two devices answer to the same loopback. You get symptoms and the required end state — no fault list. Every fault is an addressing decision that looks plausible until it is checked against the mask.

Learning objectives

• Diagnose addressing faults from operational state, with no list of what was changed
• Check an address against its mask rather than reading it as a number — including network and broadcast boundaries
• Recognise a mask mismatch on a transit link, where both ends look correctly numbered
• Spot a duplicated loopback and reason about what it breaks
• Repair faults across three routers

Troubleshooting focus

• If router-to-router pings fail: Compare masks and IPs on both ends of the shared segment. A mask mismatch can cause one side to treat the other as off-subnet even if the IPs look plausible.
• If a host cannot reach its gateway: compare its address, mask, and gateway with the router interface on that segment and confirm both sit in the same subnet, then correct whichever interface is inconsistent.
• If an interface shows administratively down: Ensure the interface is enabled and cabled correctly (no shutdown) and that each addressed interface has a real peer on the other end.
• If only one direction of ping succeeds: Re-check both ends’ addressing and masks; asymmetry often indicates one device believes the neighbor is on-link while the other does not.