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To calculate an IPv4 subnet, apply the CIDR prefix to the address: the prefix identifies the network bits, and the remaining bits define the block. For example, 192.168.1.70/26 belongs to network 192.168.1.64/26, with mask 255.255.255.192, broadcast address 192.168.1.127, and conventional host range 192.168.1.65–192.168.1.126. IPv6 also uses slash-prefix notation, but it has different address and subnet conventions; it does not use an IPv4-style dotted mask or broadcast address.
What a subnet calculator tells you
A subnet calculator takes an IP address and prefix length, then identifies the network block containing that address. For IPv4, useful results include the subnet mask, network address, total address count, broadcast address, and conventional host range. These values follow from the prefix and the address boundaries; RFC 1878 provides tables relating IPv4 prefixes, masks, address counts, and ranges (RFC 1878).
For example, in 192.168.99.0/24, the /24 means that 24 of IPv4’s 32 bits are the network prefix. The equivalent dotted-decimal mask is 255.255.255.0. CIDR prefixes, rather than the old A, B, and C address classes, determine network boundaries in modern routing (RFC 4632).
How to calculate an IPv4 network and host range
For an ordinary broadcast-capable IPv4 subnet, the first address in the block is the network address and the last is the directed broadcast address. Addresses between those boundaries make up the conventional host range.
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- Read the prefix. In
192.168.1.70/26, the prefix is/26. - Find the mask and block size. A
/26mask is255.255.255.192. It leaves 6 address bits, so each block contains 64 addresses. - Find the block containing the address. In the fourth octet, the blocks start at 0, 64, 128, and 192. Since 70 falls between 64 and 127, the address belongs to the block
192.168.1.64–192.168.1.127. - Read the boundaries. The network address is
192.168.1.64, the broadcast address is192.168.1.127, and the conventional host range is192.168.1.65–192.168.1.126.
The same approach works with other prefixes: the prefix determines the mask and block size, and the supplied address identifies which block contains it. For a /24, for instance, there are 256 total addresses in the block (RFC 4632).
How to interpret IPv4 address counts
For most conventional IPv4 subnets, the total number of addresses is not the same as the number of ordinary host addresses: the network and broadcast addresses are not assigned to hosts. The familiar “subtract two” calculation is a shortcut for those subnets, not a universal rule.
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/31point-to-point link: The block has two addresses, and both can be used as host addresses on a point-to-point link. Do not treat it as a conventional subnet with zero usable hosts (RFC 3021)./32host route: A/32represents one address as a host route; it is not an ordinary multi-address subnet with a broadcast address and host range (RFC 4632).
Likewise, advice to exclude all-zero and all-one subnets is obsolete. RFC 1878 states that the practice of excluding them is obsolete and includes all definable subnets (RFC 1878).
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IPv6 addresses are 128 bits long and written in hexadecimal groups, followed by a slash and prefix length. A prefix identifies how many leading bits belong to the network, but IPv6 does not use a dotted-decimal subnet mask, IPv4 broadcast address, or the ordinary IPv4 “subtract two” host-count method (RFC 4291).
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For globally unique and unique local address (ULA) subnets, RFC 5375 describes /64 as the subnet length and warns that other lengths can break features (RFC 5375). IPv6 addresses can have multiple valid text forms; RFC 5952 recommends a consistent representation for displaying them (RFC 5952).
What to check in a subnet calculator
When using a calculator or choosing a subnet size, check that the results match the network’s address family and intended use. In particular:
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- Confirm that the IP address and prefix are valid and belong to the intended address family.
- For IPv4, check the network and range boundaries as well as the total address count.
- Verify that the calculator handles
/31point-to-point links and/32host routes correctly. - For IPv6, look for clear prefix and address formatting rather than IPv4 mask or broadcast fields.
- For capacity planning, compare the block’s total addresses and boundaries with the number of addresses the network needs.
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