A subnet mask table maps an IPv4 CIDR prefix, such as /26, to its dotted-decimal mask and the number of addresses in the block. It helps you read subnet boundaries and host ranges; total addresses are not always the same as conventionally usable host addresses.
Quick subnet mask table: /24 through /32
These common rows show the mask, total addresses in each block, and the conventional usable-host count for ordinary IPv4 subnets.
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| 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 case |
| /32 | 255.255.255.255 | 1 | Single host route |
The conventional count subtracts the network and broadcast addresses. That shortcut describes ordinary subnet calculations, not every prefix: /31 is used for point-to-point links, and /32 denotes a single host route. See the IETF’s RFC 4632 for those distinctions.
What does the slash number mean?
The number after the slash is the count of leading 1 bits in the 32-bit IPv4 mask. Those bits identify the network or subnet portion; the remaining bits are available for the host portion. RFC 950 describes a mask as identifying the relevant bits, while RFC 4632 defines CIDR prefixes as values from 0 to 32.
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For example, 172.16.0.0/16 uses mask 255.255.0.0, and a /24 uses 255.255.255.0. A longer prefix leaves fewer host bits, so its address block is smaller.
How to calculate a subnet row with /26
-
Write 26 leading one bits, then six zero bits:
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-
Convert each eight-bit group to decimal. The mask is
255.255.255.192.Recommended Free Tools
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Find the total block size with
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-
Find the increment in the changing octet by subtracting its mask value from 256. Here,
256 − 192 = 64, so subnet boundaries in the last octet occur at 0, 64, 128, and 192. -
For each block, identify its network and broadcast addresses before finding the ordinary host range. In the RFC 1878 /26 example, blocks end at .63, .127, .191, and .255; their ordinary host ranges are .1–.62, .65–.126, .129–.190, and .193–.254. The examples appear in the IETF’s RFC 1878.
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How many usable hosts are in a subnet?
First distinguish the block’s total address count from host capacity. The total is 2^(32 − prefix length). For a conventional subnet, two addresses are set aside: the network address and the broadcast address. Thus a /26 has 64 total addresses and 62 conventional usable host addresses.
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Where to find the complete IPv4 mask table
The table above focuses on /24 through /32 for a quick lookup. The complete RFC 1878: Variable Length Subnet Table For IPv4 lists prefix-to-mask values from /1 through /32, with subnet examples. Published by the IETF as an informational RFC in December 1995, it is a historical reference; its examples include all-zero and all-one subnets under the practices cited in the memo. For modern prefix interpretation and CIDR context, consult RFC 4632, published in August 2006. Prefix size is determined by the needed address block and routing context, not by traditional class A, B, or C labels.
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