Using /31 subnets for point-to-point interfaces

Recently a “colleague”, I use that term very loosely here, was reviewing my recommendations for changes on his network. Since they’re rather tight on public IP space, and require it for a few of their PTP links, I had suggested changing them to /31 subnets. His response was less than accepting of the changes as he did everything but call me a liar.

Nonetheless, RFC 3021 first introduced the idea of thirty-one bit subnets as a means to increase the utilization of IP space in point-to-point address space. Since then most manufacturers have made a move to support the standard. Cisco’s IOS has supported the feature since version 12.2(2)T.

As for implementation, there is nothing special about the IOS configuration. An example is given below for configuring a /31 on an Ethernet segment. You’ll notice the warning, but nothing I’ve seen has given me much trouble.

R1(config)# interface fa0/0
R1(config-if)# ip address 10.0.0.0 255.255.255.254
% Warning: use /31 mask on non point-to-point interface cautiously
...  ...
R2(config)# interface fa0/0
R2(config-if)# ip address 10.0.0.1 255.255.255.254
% Warning: use /31 mask on non point-to-point interface cautiously
R2# ping 10.0.0.0

Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.0.0.0, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 11/12/18 ms