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 220.127.116.11/30 yields 18.104.22.168 and 22.214.171.124 as your usable IP addresses and 126.96.36.199 is the subnet number and 188.8.131.52 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.
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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