knowledge base

To participate in wide-area IP networking, a host needs to be configured with IP addresses for its interfaces, either manually by the user or automatically from a source on the network such as a Dynamic Host Configuration Protocol (DHCP) server. Unfortunately, such address configuration information may not always be available. It is therefore beneficial for a host to be able to depend on a useful subset of IP networking functions even when no address configuration is available.

It means link-local address are used when host is not able to get any network configuration from

• Manual Configuration
• DHCP (DHCPv4 or DHCPv6)

then it automatically take IP from pool of 169.254.0.0/16 (169.254.0.0 – 169.254.255.255) mentioned in RFC 3927

Process:

1. Take any random IP from above pool
2. Check if IP is not taken by others using broadcast via ARP
3. If IP is taken then go back to step 1
4. If IP is not taken, then assign the IP to yourself and be ready for networking.

Important points to be noted

• These address are not routable by router
  • Why : Because it’s in RFC, our rulebook for internet and also rule makes sense too.
  • In step2, we only check in our ARP domain i.e. layer 2, it makes sense not to get these routed by layer3, since there is bright chance other machines in their layer 2 would’ve done the same and IP would conflict.

Reference:

• RFC 3927