31-bit Subnet Mask (/31)
We can configure any external interface to use an IPv4 address with a 31-bit subnet mask. A 31-bit subnet mask is often used for an interface that is the endpoint of a point-to-point network. The use of 31-bit subnet masks for IPv4 point-to-point links is described in RFC 3021.


Router A is connected via Fast-ethernet 0/0 having ip to Fast-ethernet 0/0 having ip of  Router B.



R1(config)#int fa 0/0
R1(config-if)#ip add
% Warning: use /31 mask on non point-to-point interface cautiously
Thirty-one-bit subnets were first proposed in RFC 3021, which was primarily motivated by the potential for public address space conservation. Recall that shrinking a /30 subnet to a /31 effectively doubles the number of point-to-point links you can address from a finite range. Cisco IOS has supported /31 subnets for point-to-point links since release 12.2(2) T. We can put this theory into practice by addressing a point-to-point connection between two routers as  An ominous warning message, no doubt, but it works just fine. We can successfully ping the far-end interface and the subnet is accurately reflected in the routing table:

R1# ping
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echo to, timeout is 2 seconds:
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/16/20 ms

Related: Point to Point subnet

R1# show ip route is subnetted, 1 subnets
C is directly connected, FastEthernet0/0

At this juncture, one might think of using 32-bit Subnet Mask (/32) for each side of PTP links. The answer is no. Below excerpt describes why /32 IP address at each side of PTP links is not possible.
A 32-bit subnet mask defines a network with only one IP address. In mixed routing mode, you can only configure a 32-bit subnet mask for a physical external interface. Because you cannot configure a virtual external interface with a default gateway on a different subnet, you cannot use a /32 subnet mask for a virtual external interface, such as a VLAN or Link Aggregation interface.

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