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A subnet mask table maps an IPv4 CIDR prefix—such as /26—to its dotted-decimal mask, address-block size and, where applicable, conventional usable-host count. The slash number counts the mask’s leading 1 bits; a longer prefix leaves fewer addresses in the block.
Quick subnet mask table: /24 through /32
These common rows show total addresses separately from conventional usable hosts. The latter usually excludes the network and broadcast addresses, but the usual subtraction of two does not apply to every prefix.
| CIDR prefix | Dotted-decimal mask | Total addresses | Conventional usable hosts |
|---|---|---|---|
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
| /31 | 255.255.255.254 | 2 | Point-to-point special case |
| /32 | 255.255.255.255 | 1 | Single host route |
The address totals and special interpretations are consistent with the IETF’s RFC 4632. Its /31 entry is a point-to-point link, and /32 is a host route. For the complete prefix-to-mask table from /1 through /32, see the IETF’s informational RFC 1878.
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What a subnet mask table describes
A subnet mask identifies which bits of an IPv4 address belong to the network or subnet portion and which remain for host addressing; this is the function described in RFC 950. A table presents the same boundary in forms useful for different tasks: the slash prefix, the 32-bit mask in dotted-decimal notation, and often the block size and address boundaries.
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In CIDR notation, a prefix is an IPv4 address followed by a slash and a decimal number from 0 to 32. That number is the count of significant leading bits. For example, 172.16.0.0/16 uses mask 255.255.0.0; /24 corresponds to 255.255.255.0. RFC 4632 describes this notation and its prefix range.
How to read a prefix and calculate its block
- Read the prefix length. In
/26, the first 26 bits of the mask are 1s; the remaining 6 bits are 0s. - Group the mask into octets. The bits are
11111111.11111111.11111111.11000000. Converting each group of eight bits to decimal gives255.255.255.192. - Calculate total addresses. Subtract the prefix length from 32 and raise 2 to that power:
2^(32 − 26) = 2^6 = 64addresses. This is total block size, not a promise of 64 assignable hosts. - Find the increment. For this /26, the last mask octet is 192, so the increment is
256 − 192 = 64. Subnet boundaries in that octet are 0, 64, 128 and 192. - Mark each block’s network and broadcast addresses. Within a /24 divided into /26 blocks, the blocks end at .63, .127, .191 and .255. For ordinary subnet host counting, the intervening host ranges are .1–.62, .65–.126, .129–.190 and .193–.254.
The /26 boundaries and ranges above follow the examples in RFC 1878. The same approach works for other prefixes: locate the octet where the mask stops being 255, calculate its increment, and identify the network and broadcast boundaries before counting conventional usable hosts.
Total addresses are not the same as usable hosts
For an ordinary IPv4 subnet, the familiar usable-host calculation subtracts the network address and broadcast address from the total. That explains why a /26 has 64 total addresses but conventionally 62 usable host addresses. This rule is a convention for ordinary subnet calculations, not a universal property of every prefix.
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- /32: The block contains one address and represents a single host route, not a multi-host subnet.
For these edge cases, follow the addressing context and the behavior supported by the relevant platform. RFC 4632 explicitly identifies /31 point-to-point links and /32 host routes.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to use a table when choosing a prefix
Start with the number of addresses the subnet needs, then consider routing and the addressing context. The prefix determines the block size: increasing the prefix by one halves the number of total addresses. Do not use the historical class A/B/C labels as the basis for modern prefix selection; CIDR prefixes express the boundary directly. RFC 1878 is an informational table published in December 1995, and its examples include all-zero and all-one subnets under the practice described by that memo. For current network configuration, check the applicable routing plan and device behavior rather than treating a historical table as a configuration policy.
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