Monday, July 7, 2014

A special case in IPV4 subnetting /31 prefixes

Lack of IPV4 addresses pushed think-tanks to come-up with various optimization techniques. /31-bit prefixes for IPV4 point-point link is one such optimization. This is a special case in IPV4 sub-netting.

A basic thumb-rule in subnet masking is,

  1. 1st address to network
  2. last address to Broadcast
  3. IP address in between are usable IPs for hosts

For Example: prefix: Network:, Broadcast: Hosts:

Hence for any IP subnet 2 IP addresses used to make the subnet work. 

When i started my carrier as old timer in IP networking, i used to use only /30 for P2P links. I have generously wasted 2 ips per PTP link. Internet consortium came up with a special case to mitigate this IP address wastage. RFC3021 has more information about /31-but prefix

Latest Routers accept /31 as valid subnet. In a PTP network Broadcast and network IP has less value. Router will forward to peer if it identifies dst.ip belongs to his subnet but not belong to him. 

All VLSM (Variable Length Subnet Masking) supported routing protocols understands /31. 

Hope you learned something new. Happy Networking!

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