Subnet calculator (IPv4 and IPv6 CIDR)

A /24 has 256 addresses and 254 usable hosts, because the all-zeros address names the network and the all-ones address is the broadcast. The IPv4 rule is 2^(32 − prefix) − 2: a /25 gives 126, a /26 62, a /27 30, a /28 14, a /29 6 and a /30 2. The exceptions are /31, where RFC 3021 lets a point-to-point link use both addresses, and /32, a single host.

They say the same thing in two ways. CIDR notation writes the number of network bits after a slash, as in /26; a subnet mask writes those bits as a dotted address with that many leading ones, so /26 is 255.255.255.192 and /20 is 255.255.240.0. CIDR, introduced in 1993 and now defined in RFC 4632, replaced the fixed class A, B and C boundaries, so any prefix from /0 to /32 is valid.

A wildcard mask is the subnet mask with every bit inverted, so 255.255.255.192 becomes 0.0.0.63. Cisco IOS access lists and OSPF network statements use it: a 0 bit must match and a 1 bit is ignored. To get it, subtract each octet of the netmask from 255. For a /26 the last octet is 63, one less than the block's 64 addresses.

Both clouds reserve 5 addresses in every subnet: the network address, the next three (for the router and DNS, or held for future use) and the last address. A /24 in an AWS VPC or Azure virtual network therefore has 251 assignable addresses instead of the 254 shown here. The smallest subnet AWS allows is a /28, which leaves 11; Azure's smallest is a /29, which leaves 3.

A /64 holds 2^64 = 18,446,744,073,709,551,616 addresses. It is the standard size for one IPv6 LAN because RFC 4291 requires 64-bit interface identifiers for most unicast addresses, and stateless autoconfiguration (SLAAC) builds addresses from them. A /48 contains 65,536 of those /64 subnets, and a /56 contains 256.

Accuracy depends on your inputs and the method's assumptions. Decimal arithmetic uses 50 significant digits, but estimates, numerical methods and source data can be less precise; the displayed rounding does not remove those limits. It is checked against 7 worked examples whose answers come from independent sources; for example, “Host in a /26” is checked against Python 3.8 ipaddress.ip_interface('192.168.10.77/26').network, .netmask, .hostmask, .broadcast_address, list(hosts()).

RFC 4632 — Classless Inter-domain Routing (CIDR); RFC 3021 — Using 31-bit prefixes on IPv4 point-to-point links; RFC 1918 — Address allocation for private internets; RFC 4291 — IPv6 addressing architecture; RFC 5952 — IPv6 text representation; IANA IPv4 and IPv6 special-purpose address registries (RFC 6890).