Friday, April 27, 2012

Configuring /31 - P2P or Segment with 2 endpoints

In general, most ISPs will assign a /30 on point-to-point interfaces which gives you 4 IP addresses and 2 of which are usable resulting in 50% of addresses in that subnet being wasted. For example yields and as your usable IP addresses and is the subnet number and is the broadcast address. And as you can see only 2 out of the 4 IP addresses are usable. So how we can be more efficient? The use of a /31 subnet mask which gives you 2 IP addresses and 100% of them will be utilized.

Consider a very simple topology below and let's get started.
 As far as implementing the /31 on interfaces it's straight forward.

Notice that everything is pingable and working as it should. Generally speaking /31 is assigned on point-to-point interfaces, however you can use it on broadcast interfaces such as Fast Ethernet.

Notice the 'Warning' message.

Official RFC is RFC 3021 which you find here.

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