/31 Subnet: Mask 255.255.255.254
A /31 subnet has a subnet mask of 255.255.255.254 and contains 2 addresses, both usable as a point-to-point link (RFC 3021). 128 /31 networks fit inside a single /24.
| CIDR notation | /31 |
|---|---|
| Subnet mask | 255.255.255.254 |
| Wildcard mask | 0.0.0.1 |
| Mask in binary | 11111111.11111111.11111111.11111110 |
| Total addresses | 2 (2^1) |
| Usable hosts | 2 |
| Host bits | 1 |
Usable hosts in a /31 by platform
Cloud providers reserve more than the usual two addresses per subnet, so the same /31 gives you fewer usable IPs in the cloud than on a standard LAN.
| Platform | Reserved IPs | Usable hosts |
|---|---|---|
| Standard (network + broadcast) | 0 | 2 |
| AWS VPC | 0 | 2 |
| Microsoft Azure VNet | 0 | 2 |
| Google Cloud VPC | 0 | 2 |
| Oracle Cloud VCN | 0 | 2 |
Good to know
A /31 (RFC 3021) gives exactly 2 usable addresses with no network or broadcast reservation, designed for point-to-point links to save address space.
How a /31 is calculated
- The /31 means the first 31 bits are the network portion, leaving 1 host bits.
- Total addresses = 2^(32 - 31) = 2^1 = 2.
- Usable hosts (standard) = 2 = 2.
- Subnet mask = 255.255.255.254; wildcard mask = 0.0.0.1.
Want to split a /31 into smaller subnets, see every address range, or plan a VLSM layout? Open the free subnet calculator →