Duplicate IP and MAC Address Issues

Duplicate IPs

two systems could have same IP address if:
- configuration error
  - statically assigned
  - address part of DHCP scope
Windows displays a warning and disable the IP if it detects duplicates
Linux does not disable an interface when there is a duplicate IP
- may log a warning
- a race condition determines which host receives traffic
Fixing duplicates:
- obtain MAC addresses of both interfaces
  - use ping then arp -a to examine ARP cache table
  - Linux: arping -D to report duplicate replies
- once identified, configure to unique address

duplicate MAC address causes both hosts to contend to respond to ARP queries
- communications could be split between them or reach only one host
- unlikely to arise unless the network uses locally administered addressing
diagnose MAC address issues:
- verify the MAC addresses recorded for each host
  - use arp
- check the MAC address assigned to the interface
  - use ifconfig or ip neigh
- check MAC addresses and ARP tables on switches and routers involved in communications path
- use a protocol analyzer to examine ARP traffic and identify which IP hosts are attempting to claim the same MAC address