Ok, this is strange! At least this was my first reaction when I saw that in one of my CCIE labs that I am trying to resolve all the links between routers are addresses with a subnet /31.
Isn’t that weird that something like this you see for this first time after couple of years in networking. For me it was. It blow my mind out. I asked my more experienced networking colleagues later but for them it seemed new too. They said at first: Ok men, that’s not possible!
Well, try to type it on router interface and you will se that it is possible. It strange for sure, but it’s possible. Router OS (Cisco IOS in this case) will try to be sure that you will use this kind of subneting only for Point-to-point links. That’s why it will issue a warning message if you apply this subnet mask on an Ethernet interface. For serial it will go without the warning.
The idea behind this is of course simple if you put it this way: