What is a /31 subnet?
2 usable hosts out of 2 addresses, behind netmask 255.255.255.254.
| Prefix | /31 |
|---|---|
| Netmask | 255.255.255.254 |
| Total addresses | 2 |
| Usable hosts | 2 |
| Example block | 10.0.0.0/31 |
| Usable host range | 10.0.0.0 – 10.0.0.1 |
A /31 fixes the first 31 bits of the address as the network and leaves 1 for hosts — 2 addresses in all.
A /31 is the exception. It has no network or broadcast address, so both of its addresses are usable — which is exactly what a point-to-point link between two routers needs (RFC 3021).
128 of them fit inside a /24.
Other subnet sizes
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