/31 subnet (255.255.255.254)
A /31 network has 2 addresses. Two addresses with no network or broadcast address (RFC 3021). Used for point-to-point links between routers.
- CIDR notation
- /31
- Subnet mask
- 255.255.255.254
- Wildcard mask
- 0.0.0.1
- Binary mask
- 11111111.11111111.11111111.11111110
- Total addresses
- 2
- Usable hosts
- 2
- Example network
- 192.168.1.0/31
- Example usable range
- 192.168.1.0 – 192.168.1.1
- /31 networks in a /24
- 128
Calculate a /31 for your own address
In firewall rules and investigations
A /31 rule matches every address from 192.168.1.0 to 192.168.1.1. In Cisco ACLs the same range is written with the wildcard mask 0.0.0.1. When an alert names an IP, checking whether it falls inside a known /31 (your own network, a cloud provider’s published range or a blocked range) is often the quickest first step.