/31 Subnet Mask Reference (255.255.255.254)
Modern RFC 3021 point-to-point router interconnect link (2 usable hosts, zero wasted broadcast).
| CIDR Prefix | /31 |
|---|---|
| Subnet Mask | 255.255.255.254 |
| Wildcard Mask (Cisco / ACL) | 0.0.0.1 |
| Total IPv4 Addresses | 2 |
| Usable Host IPs | 2 |
| Binary Subnet Mask | 11111111.11111111.11111111.11111110 |
| Example Block (10.0.0.0/31) | 10.0.0.0 – 10.0.0.1 |
| Example Usable Hosts | 10.0.0.0 – 10.0.0.1 |
Need to compute custom IP ranges? Try the Interactive Subnet Calculator or VLSM Splitter.
Understanding the IPv4 /31 CIDR Block
An IPv4 /31 prefix allocates 31 network bits and 1 host bits out of a 32-bit IPv4 address, yielding an address block of 2^1 = 2 total IPv4 addresses. Expressed in dotted-decimal notation, the subnet mask is 255.255.255.254, and its bitwise inverse—used in Cisco IOS access control lists (ACLs) and OSPF area statements—is wildcard mask 0.0.0.1.
Under RFC 3021, a /31 prefix eliminates directed broadcast semantics specifically for point-to-point router interconnects, allowing both IP addresses in the 2-address block to be assigned directly to router interfaces with zero address waste.