IPv4 /31 vs /32: differences, hosts, and use cases

Short answer

A /31 contains two addresses and can use both as the endpoints of an IPv4 point-to-point link, without reserving separate network and broadcast addresses. A /32 contains exactly one address: it represents a host route, not a subnet with a second device to connect to.

Direct comparison

The special /31 interpretation is limited to point-to-point links. It does not turn any two-address shared LAN into an appropriate network for arbitrary devices.

A /31 example

For 192.0.2.10/31, the host bit produces two values:

192.0.2.10
192.0.2.11

On a compatible point-to-point link, both identify the two endpoints. There is no third address and neither value is reserved as directed broadcast. The IPv4 subnet calculator recognizes this mode, lists both addresses as usable, and marks broadcast as not applicable.

A /32 example

192.0.2.10/32 selects only 192.0.2.10. All 32 bits form the prefix, so no host bits remain to number a range inside the block.

A /32 route may be used to reach one address, advertise a loopback, or express a policy applying to one source or destination. It does not provide a neighbor address or a broadcast range.

Why “total minus two” fails

The traditional rule reserves the all-zero host value for the network and the all-one value for broadcast. In a /31, those are the only two possible values. RFC 3021 redefines both as hosts on a point-to-point link to avoid consuming four addresses for two endpoints.

In a /32, there is no host portion. Its only address identifies the complete route. Subtracting two would produce negative capacity, which signals that the formula is outside its valid scope.

Compatibility checks

• Confirm that both endpoints and networking equipment support RFC 3021 /31.
• Use /31 only when the medium is genuinely point to point.
• Review interface behavior instead of assuming shared-LAN discovery or broadcast semantics.
• Check a provider’s policy before configuring the addresses.
• Distinguish an interface address from a /32 route in the routing table.

Frequent mistakes

• Calling the second address of a point-to-point /31 the broadcast address.
• Using /32 while expecting two devices to communicate as subnet peers.
• Assigning /31 to a multi-access network without checking support.
• Reporting zero hosts because a calculator always applies 2^h - 2.
• Confusing a route prefix with the mask configured on an interface.

RFC 3021 specifies that both addresses in a /31 are interpreted as hosts for point-to-point use. RFC 4632 classifies /32 as a host route and /31 as a point-to-point link.