The /31 subnet, explained
Everything about a /31 CIDR block — mask, address count, usable hosts and where a /31 fits in real network plans.
| Prefix length | /31 — 31 network bits, 1 host bits |
| Subnet mask | 255.255.255.254 |
| Wildcard (ACL) mask | 0.0.0.1 |
| Binary mask | 11111111.11111111.11111111.11111110 |
| Total addresses | 2^1 = 2 |
| Usable hosts | 2 (RFC 3021 point-to-point — both addresses usable) |
| A /16 holds | 32,768 × /31 subnets |
| Example block | 192.168.1.0/31 |
Notes
A /31 prefix means the first 31 bits of the 32-bit IPv4 address identify the network and the remaining 1 bits identify hosts — giving 2^1 = 2 addresses. RFC 3021 makes both addresses of a /31 usable on point-to-point links.
Cloud sizing: too small for an AWS VPC (minimum /28 per VPC, /29 allowed for GCP subnets; /31 and /32 are for point-to-point links and host routes, not subnets).
Adjacent sizes: /8 · /9 · /10 · /11 · /12 · /13 · /14 · /15 · /16 · /17 · /18 · /19 · /20 · /21 · /22 · /23 · /24 · /25 · /26 · /27 · /28 · /29 · /30 · /32
Frequently asked questions
How many IP addresses are in a /31?
2 total addresses, of which 2 are usable hosts.
What is the subnet mask for /31?
255.255.255.254 (wildcard mask 0.0.0.1).
What does /31 mean in an IP address?
The /31 suffix is CIDR notation: the first 31 bits are the network part, the last 1 bits are the host part.