IntermediatePublished 2026-07-04
Secondary IP on a LAN Interface
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Lab 5 of 10 in IP Addressing & Subnetting · ← Previous · Next →
CCNA exam domain: Network Fundamentals
Configure a Cisco IOS router to host two IPv4 /24 subnets on a single physical LAN interface using a secondary address. Verify directly-connected reachability only (no routing protocols, no static routes). This simulates a readdressing coexistence period where both old and new subnets must operate concurrently on the same segment.
Learning objectives
- Explain when and why to use a secondary IPv4 address on a single LAN interface
- Configure a primary and a secondary IPv4 /24 on the same IOS interface using correct command order
- Verify interface addressing using show commands and confirm directly-connected pings
- Recognize common pitfalls: mask mismatch, missing secondary keyword, or wrong default gateway on hosts
Troubleshooting focus
- Primary/secondary order: ensure the primary address line is configured first, followed by the secondary line
- Mask accuracy: both subnets must use 255.255.255.0 and must not overlap
- Host gateway alignment: each host must use the gateway that matches its own subnet
- Interface state: verify Ethernet0/1 is up/up and not administratively down
- ARP sanity: check ARP table entries appear for hosts in both subnets after pings
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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