IPv4 Subnet Calculator - CIDR, Netmask, Wildcard, Host Range and Subnet Splitter

An IPv4 address is a 32 bit number written as four bytes. A prefix length such as /24 says how many leading bits name the network. The rest identify hosts on it. The netmask is that many 1 bits followed by 0 bits (/24 is 255.255.255.0), and the wildcard mask, which some router configurations use, is the same mask turned upside down (0.0.0.255). The network address is the address AND the netmask. The broadcast address is the network address OR the wildcard mask. A prefix of length n holds 2 to the power of (32 - n) addresses: 256 for a /24, which is 254 usable hosts once the network and broadcast addresses are taken out. This notation is CIDR (RFC 4632). It replaced the old classes A, B and C that RFC 791 defined by the first bits of an address. The tool still shows the class for reference.

Two prefixes break the minus-two rule. RFC 3021 lets a point-to-point link use a /31: both addresses are hosts, with no network or broadcast address. A /32 is a single address, used as a host route or to name one device. The tool shows a /31 as 2 usable hosts and a /32 as 1. You can type the size as /24, as a netmask (192.168.1.10 255.255.255.0) or as a wildcard mask (192.168.1.10 0.0.0.255). A mask is read as a netmask if it is a valid one, and as a wildcard mask if only that fits. 0.0.0.0 and 255.255.255.255 fit both ways, so Detect reads them as netmasks and you can switch it. If the address has host bits set, as 192.168.1.10/24 does, the tool shows the network that contains it.

The address type comes from the IANA IPv4 special-purpose address registry. The private ranges of RFC 1918 are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Loopback is 127.0.0.0/8 (RFC 1122). Link-local addresses, which a device gives itself when it has no other configuration, are 169.254.0.0/16 (RFC 3927). 100.64.0.0/10 is the shared address space used for carrier-grade NAT (RFC 6598). 224.0.0.0/4 is multicast (RFC 5771) and 240.0.0.0/4 is reserved for future use (RFC 1112). The documentation ranges 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24 are for examples (RFC 5737), and 198.18.0.0/15 is for benchmarking (RFC 2544). An address in none of these blocks is shown as ordinary public space, which does not mean that anyone has been allocated it.

Splitting into equal subnets adds bits to the prefix. Each extra bit doubles the number of subnets and halves their size, so a /24 split into four gives four /26 blocks of 64 addresses and 62 usable hosts. The hosts-needed mode is variable length subnet masking (VLSM). Each requirement is rounded up to the smallest block that holds that many hosts plus the network and broadcast addresses, so 100 hosts need 102 addresses, which means a /25 of 128. The blocks are placed from the start of the network, biggest first, so each sits on its natural boundary, and what is left over is listed as the fewest CIDR blocks.

The range to CIDR mode finds the fewest CIDR blocks that cover exactly the addresses from the first to the last. At each step it takes the biggest block that starts at the current address, sits on its own size boundary and does not run past the end. A range rarely lines up with one prefix: 192.168.1.5 to 192.168.1.200 needs nine blocks. The arithmetic uses unsigned 32 bit numbers throughout, so addresses above 127.255.255.255 are handled correctly. IPv6 is not covered.