/31 subnet
A /31 has a subnet mask of 255.255.255.254 and 2 usable hosts.
The numbers
| Prefix length | /31 |
| Subnet mask | 255.255.255.254 |
| Wildcard mask | 0.0.0.1 |
| Network bits | 31 |
| Host bits | 1 |
| Total addresses | 2 |
| Usable hosts | 2 |
| /24 networks contained | — |
Where a /31 is used
Two addresses, both usable. Under the ordinary rule a /31 would have no assignable hosts at all; RFC 3021 makes an exception for point-to-point links, where a broadcast address serves no purpose. It halves the waste of using a /30.
Why the usable count is what it is
Following the ordinary rule, a /31 would have two addresses and lose both to the network and broadcast, leaving nothing. RFC 3021 recognises that a point-to-point link has exactly two endpoints and no need to broadcast, so on such a link both addresses are assignable. This is why modern router configurations use /31 where older ones used /30.
Binary layout
mask 11111111.11111111.11111111.11111110
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
31 network bits, 1 host bitsSplitting and combining
| Prefix | Usable hosts | Relationship | |
|---|---|---|---|
| Twice the size | /30 | 2 | Two /31 networks combine into one /30 |
| This network | /31 | 2 | — |
| Half the size | /32 | 1 | One /31 splits into two /32 networks |
Frequently asked questions
How many hosts are in a /31?
2 addresses in total, of which 2 are usable. Both are usable because RFC 3021 removes the broadcast address on point-to-point links.
What is the subnet mask for /31?
255.255.255.254. The prefix length is the count of leading 1 bits in the mask, so /31 is 31 ones followed by 1 zero.
What is the wildcard mask for /31?
0.0.0.1 — the bitwise inverse of the subnet mask. Cisco access lists and OSPF network statements take the wildcard rather than the mask; a zero bit must match and a one bit is ignored.
How does a /31 compare to a /24?
A /31 is smaller: 128 of them fit inside one /24.