What are common IP addressing issues and how can they be identified?

Caused by manual mistakes or DHCP issues; verify with show ip interface brief (IOS) or ipconfig (Windows). A 169.254.x.x (APIPA) address means the host couldn't reach a DHCP server.

IP Addressing Issues:

Two common causes of incorrect IPv4 assignment:

1. Manual assignment mistakes
2. DHCP-related issues

On IOS Devices:

Use show ip interface or show ip interface brief to verify what IPv4 addresses are assigned to network interfaces.

On Windows End Devices:

APIPA (Automatic Private IP Addressing):

• When a Windows device cannot contact a DHCP server, it automatically assigns an address in the 169.254.0.0/16 range
• A computer with an APIPA address cannot communicate with other devices on the network (other devices won't be in that range)

Note: Linux and OS X do not use APIPA.

Verification commands:

ipconfig                   # Windows
ifconfig                   # Linux/Mac
show ip interface brief    # Cisco IOS

Key insight: If you see a 169.254.x.x address on a Windows machine, it indicates a DHCP problem - either the DHCP server is unreachable or not responding.

Go deeper:

• Wikipedia — Link-local address (APIPA) — why the 169.254.0.0/16 range is self-assigned when no DHCP server answers, and why such a host can't reach others.
• Wikipedia — DHCP — the DORA exchange that should have handed out a real address.