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 1.1.1.0/30 yields 1.1.1.1 and 1.1.1.2 as your usable IP addresses and 1.1.1.0 is the subnet number and 1.1.1.3 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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