Secondary IP on a LAN Interface

Bundle lab · $29.99

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