Subnet Calculator

Reading the result

• Network — the first address, with every host bit zero, and the prefix. An address typed with host bits set, such as 10.1.2.3/8, is kept on its own row and its network worked out.
• Broadcast — the last address, with every host bit one. IPv6 has none.
• First and last usable — the addresses hosts can take: all but the network and broadcast address, so a /24 has 256 addresses and 254 usable hosts.
• Subnet mask and wildcard mask — the prefix as a dotted mask, and its inverse, which Cisco ACLs and OSPF take.
• Reverse DNS zone — the in-addr.arpa or ip6.arpa zone that holds the network's PTR records. A single address shows its own PTR name.
• Address type — the line under the network: the IANA special-purpose entry the whole network falls in, and whether the registry marks it globally reachable. A network that spans several entries lists the ones it contains.

A mask after the address is read as a subnet mask when it is one, and otherwise as a wildcard mask. 0.0.0.0 and 255.255.255.255 are both, and are read as subnet masks (/0 and /32), with a note. An address without a prefix is a single address, /32 or /128.

How it works

An address is a 32-bit (IPv4) or 128-bit (IPv6) number. The prefix length is how many leading bits name the network; the rest number the hosts inside it. All arithmetic is done on whole numbers of that size, so an IPv6 /0 is counted exactly.

A /31 holds only two addresses. RFC 3021 lets both be hosts on a point-to-point link, so the tool shows two usable hosts and no broadcast; a /32 is one host. IPv6 /127 links follow RFC 6164.

IPv6 addresses are written in the RFC 5952 form: lowercase, leading zeros dropped, and the longest run of two or more zero groups shortened to :: (the first, when two runs tie). The expanded row writes all eight groups.

VLSM sizes each subnet for its host count, the largest first, and packs them from the start of the network, so every subnet starts on a boundary of its own size. An IPv4 subnet keeps its network and broadcast addresses, so 1 or 2 hosts take a /30 and 3 a /29. /31 for 2-host links, off by default, gives a need of 1 or 2 hosts a /31 instead, as RFC 3021 allows on point-to-point links. VLSM never gives a /32.

network = address AND mask

broadcast = network OR wildcard

total addresses = 2^(32 − prefix)

usable hosts = total − 2 (a /31 has 2, a /32 has 1)

Examples

IPv4 prefix cheat sheet

Address types and the standards

Address types come from the IANA IPv4 and IPv6 Special-Purpose Address Registries as last updated on 9 October 2025, with each entry's Globally Reachable flag, the blocks of the IPv4 Multicast Address Space and the top-level IPv6 allocations. IPv6 multicast addresses also show their scope (RFC 7346). A special-purpose block is not always private: the AS112 and anycast blocks are globally reachable. Any IPv4 address outside every entry is public.

The private ranges of RFC 1918 are 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16; 100.64.0.0/10 is shared address space for carrier-grade NAT (RFC 6598), and fc00::/7 holds IPv6 unique local addresses.

Sources

• IANA IPv4 Special-Purpose Address Registry — IANA
• IANA IPv6 Special-Purpose Address Registry — IANA
• IPv4 Multicast Address Space Registry — IANA
• Internet Protocol Version 6 Address Space — IANA
• RFC 4632: Classless Inter-domain Routing (CIDR) — IETF
• RFC 1918: Address Allocation for Private Internets — IETF
• RFC 2317: Classless IN-ADDR.ARPA delegation — IETF
• RFC 3021: Using 31-Bit Prefixes on IPv4 Point-to-Point Links — IETF
• RFC 4291: IP Version 6 Addressing Architecture — IETF
• RFC 5952: A Recommendation for IPv6 Address Text Representation — IETF
• RFC 6164: Using 127-Bit IPv6 Prefixes on Inter-Router Links — IETF
• RFC 6598: IANA-Reserved IPv4 Prefix for Shared Address Space — IETF
• RFC 7346: IPv6 Multicast Address Scopes — IETF
• RFC 9637: Expanding the IPv6 Documentation Space — IETF

Reviewed 20 October 2026

Frequently asked questions

See also

• Number Base Converter — to convert an address or mask between binary, decimal and hex.