IntermediatePublished 2026-07-04
Point-to-Point Links with /30 and /31
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Lab 4 of 10 in IP Addressing & Subnetting · ← Previous · Next →
CCNA exam domain: Network Fundamentals
Build confidence addressing IPv4 point-to-point WAN links on Cisco IOS using /30 and /31 masks. Two routers are connected by two parallel links: one classic /30 and one RFC 3021 /31. No routing protocols or static routes are configured — the goal is deterministic, correct interface addressing and verification of directly connected reachability. A small LAN off R1 with two hosts allows additional verification (host-to-gateway only).
Learning objectives
- Assign correct IPv4 /30 and /31 addresses to router-to-router point-to-point links
- Explain address utilization differences between /30 and /31 on P2P media
- Verify link state and directly connected reachability without any routing configured
- Use show commands to confirm addresses and masks on the intended interfaces
Troubleshooting focus
- Mask mismatch on a point-to-point link (/30 on one side, /31 on the other) prevents ARP/neighbor resolution and pings
- Using the network or broadcast address on a /30 (198.51.100.0 or .3) results in an interface that appears up but cannot pass IP traffic
- Forgetting no shutdown leaves the link down/down even with correct IP addressing
- Assigning both ends the same IP or swapping intended higher/lower address across the pair breaks reachability
- Applying the address on the wrong physical interface (Gi0/2 vs Gi0/0 or Gi0/1) causes unexpected failures
Topology
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How this lab is graded
- Build it your way. Where a lab lets you choose a value — a VLAN name, an interface description — grading checks that you configured it, not which name you picked. Names that another line has to reference, like an ACL applied with
access-class, are stated in the guide and do have to match. - Addresses, modes and protocol keywords are exact. An IP address, a subnet mask,
switchport mode trunk, an encapsulation — these carry the meaning of the lab, so they are graded as written in the guide. - Grading reads your saved configuration. Export the lab from CML after you have configured it, and make sure anything you set is in the
running-config— a change that only exists in a terminal session never reaches the grader. - You can submit as many times as you like. Your best score stands, and each attempt tells you which checks passed so you can work the gaps.
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