Why /30 why not /31?

May be a year ago, I was reading a document in which /31 subnet addressing was used for a point-to-point link. The document didn’t talk much about it as it was written for something else. The /31 interested me. I started thinking about it. Why /30 why not /31?

What impact will it make if we use /31 for addressing a point-to-point link? I could imagine nothing. With /31 there won’t be a network and a broadcast address, we will be having two host addresses only. Do we really need a network and broadcast addresses for point-to-point link? It looks actually not. All we have is just two network end points or two hosts connected to each other with nobody else. There is no need for a broadcast in this network.

Choosing a /31 will leave us with no directed broadcast address for that network which is anyways of no need in a point-to-point link.

Google’ing on the same showed Cisco started supporting /31 since IOS 12.2(2)T and usage of /31 on point-to-point links has been well discussed in RFC 3021 (published in Dec-2000, oh man!! How many years am I lagging). Juniper and other vendors do support this.

What do we achieve by starting to use /31? – Well dude, you save two IPv4 addresses per link. If you are an ISP, especially in APNIC, you might have already be facing tough time in getting new IPv4 address blocks. We are almost out of any free subnets. If you have 1000 links already in /30 then by switching to /31 you can provision to 1000 more links, means 1000 more customers.

Until the entire world in ready for IPv6, IPv4 addresses are gold dude, spend them as little as you can. Next time you reserve a subnet for addressing point-to-point link, do remember to do /31 not /30.

TIPS:

If you are an engineer who often needs to find the address of the circuit interface at the other end of a /30 by looking at your end IP address and you are doing the binary/math calculation every time, next time you don’t have to, in a /30 subnet the lower address will always be odd and the higher address will always be even.

e.g.: If 192.168.10.10/30 is what is assigned to your end of the link, by looking at it - the last octet .10 is even - the other end IP address will be 192.168.10.9 which is one lower from 192.168.10.10

Up on reading this post, if you have decided to use /31 going forwardJ, then the rule for calculating the other end IP just reverses. In /31 subnet the lower address will always be even and the higher address will always be odd.