Can someone tell me if /31 subnet mask be used anywhere?

[grapevine]

Can someone tell me if /31 subnet mask be used anywhere? Because we have only two IP addresses and I saw in one of the training videos that both of them can be assigned to hosts. Please clarify

[Giuseppe Larosa]

Hello Ranji,

/31 subnets have been introduced some years ago to help with the shortage of IPv4 addresses.

The introduction is described in the following RFC

https://tools.ietf.org/html/rfc3021

So yes they are allowed and in use.

Hope to help

Giuseppe

[Mark Malone]

Hey this may help , never used them personally but they can be used ietf doc describes it more why there used

Using 31-Bit Prefixes on IPv4 Point-to-Point Links RFC 3021 leavingcisco.com describes using 31-bit prefixes for point-to-point links. This leaves 1 bit for the host-id portion of the IP address. Normally a host-id of all zeros is used to represent the network or subnet, and a host-id of all ones is used to represent a directed broadcast. Using 31-Bit prefixes, the host-id of 0 represents one host, and a host-id of 1 represents the other host of a point-to-point link. Local link (limited) broadcasts (255.255.255.255) can still be used with 31-bit prefixes. But directed broadcasts are not possible to a 31-bit prefix. This is not really a problem because most routing protocols use multicast, limited broadcasts, or unicasts.

http://tools.ietf.org/html/rfc3021