Calculate an IPv4 or IPv6 subnet

What a subnet calculator does

A subnet is a block of consecutive IP addresses. CIDR notation describes one with an address and a prefix length, for example 192.168.1.0/24. The number after the slash says how many bits at the start of the address identify the network. The remaining bits identify hosts inside it.

The calculator turns that short notation into the details you need when planning or debugging a network: the network address, the broadcast address, the first and last usable host, how many hosts fit, and the netmask in dotted form.

Reading the IPv4 result

• Network address: the first address in the block, which names the subnet itself.
• Broadcast address: the last address, used to reach every host on the subnet.
• Usable hosts: the addresses between them. A /24 has 256 addresses and 254 usable hosts.
• Netmask and wildcard: the same boundary written two ways. The wildcard is the netmask turned inside out and is used in some router access lists.
• Binary view: the address and mask in bits, where you can see exactly where the network part ends.
• Class and flags: the historical class A to E, and whether the block is private, loopback, link-local, shared CGNAT space or public.

Special cases

A /32 is a single address. A /31 holds two addresses with no network or broadcast, so both can be used, which suits links between two routers. A /0 covers the whole internet.

If you type a host address such as 172.16.5.4/20, the calculator shows the network it belongs to, 172.16.0.0/20, because the host bits are cleared.

IPv6 prefixes

IPv6 addresses are 128 bits, and networks are sized differently. A single network, such as a home LAN, normally gets a /64, which holds more than 18 quintillion addresses. Providers usually hand customers a /56 or /48, which contain 256 or 65,536 of those /64 networks. The IPv6 calculator shows the first and last address of a prefix and how many /64s it holds. Everything runs in your browser, and nothing you enter is sent anywhere.

Common uses

• Planning how to divide a private network into separate subnets for offices, servers or guests.
• Checking which addresses a firewall or access rule actually covers.
• Working out the smallest block that fits a number of devices.
• Converting between a prefix length such as /20 and a netmask such as 255.255.240.0.
• Confirming whether an address falls inside a block your provider has assigned.