Read the prefix before counting hosts
IPv4 addresses contain 32 bits. A /24 prefix leaves eight host bits and therefore 256 total addresses in that block. A conventional broadcast subnet often reserves the lowest address as the network identifier and the highest address as broadcast, leaving 254 addresses for interfaces. This familiar subtraction does not apply uniformly to all prefixes or link types.
Start with network boundaries: an address such as 192.0.2.19/27 belongs to the block 192.0.2.0 through 192.0.2.31. The next /27 begins at .32. Prefix arithmetic answers which addresses are in a block; interface eligibility and directed-broadcast behavior depend on the network design.
Why /31 is special
RFC 3021 describes using 31-bit prefixes on point-to-point IPv4 links. With a single host-number bit, there are exactly two addresses. On a supported point-to-point link, both addresses identify endpoints rather than following the ordinary network/broadcast reservation rule.
For example, 192.0.2.10/31 covers 192.0.2.10 and 192.0.2.11. It is an efficient allocation for a routed point-to-point adjacency, but using it on an arbitrary LAN segment requires careful attention to platform and topology support. It is not a general recommendation for every Ethernet broadcast network.
What /32 means in practice
A /32 names exactly one IPv4 address. It is frequently used for host routes, loopback interfaces, policy rules and endpoint identifiers. A /32 does not have a second usable peer address within the same prefix. Reachability still depends on routing, neighbor discovery where applicable, or an explicit point-to-point configuration.
A common mistake is interpreting an address count as a connectivity guarantee. A subnet calculator reports mathematical boundaries, not whether a particular device is online.
A repeatable subnet verification workflow
- Confirm the input IP address and prefix; reject octets outside 0–255 and prefixes outside 0–32.
- Calculate the network mask and align the address to its prefix boundary.
- Identify the network's first and last address, then determine whether broadcast reservations apply to this topology.
- When splitting a network, verify the subnets remain within the original allocation and do not overlap other reserved ranges.
- When aggregating blocks, confirm that combining routes will not advertise space you do not control.
Use the site's IPv4 Subnet Calculator for boundaries and CIDR Split & Aggregate for allocation planning. Cross-check router/interface constraints before changing a production addressing plan.