IP Subnet Calculator for IPv4 CIDR and IPv6

IPv4 prefix lengths, masks and host counts

Special IPv4 address ranges

IPv6 address types

Common IPv6 prefix sizes

How to calculate a subnet

1. Write the address with its prefix. Type the IPv4 address followed by a slash and the prefix length, like 192.168.1.10/24, or follow the address with a dotted netmask like 255.255.255.0. The prefix is how many leading bits name the network.
2. Find the network and broadcast addresses. Keep the network bits and set the host bits to 0 for the network address (192.168.1.0), or to 1 for the broadcast address (192.168.1.255).
3. Count the usable hosts. A /24 leaves 8 host bits, so 2^8 = 256 addresses. Take away the network and broadcast addresses and 254 are left for hosts, from 192.168.1.1 to 192.168.1.254. A /31 keeps both of its 2 addresses and a /32 is one host.
4. Split into smaller subnets. Each extra prefix bit halves the block. Choose how many subnets you need (a power of 2) or the prefix you want: 192.168.1.0/24 into 4 subnets gives 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/26.
5. Check the address type. The calculator names private, carrier-grade NAT, loopback, link-local, multicast and documentation ranges, so you can see at once whether an address is meant to be routed on the internet.

Frequently asked questions

How do I calculate the network address from an IP address and prefix?

Write the address in binary and set every host bit (the bits after the prefix) to 0. For 192.168.1.10/24 the first 24 bits are the network, so the network address is 192.168.1.0. In decimal terms, round the address down to a multiple of the block size: a /24 block is 256 addresses long.

How many usable hosts are in a /24, /26 or /30?

A /24 has 254 usable hosts, a /26 has 62 and a /30 has 2. The formula is 2^(32 − prefix) − 2: the two taken away are the network address and the broadcast address.

How are /31 and /32 networks counted?

A /31 has 2 addresses and both are usable: RFC 3021 lets point-to-point links drop the network and broadcast addresses, so a router link needs 2 addresses instead of 4. A /32 is a single host address with 1 usable host and no broadcast. The calculator shows both exactly this way.

What is a wildcard mask?

The wildcard mask is the netmask with every bit flipped: 255.255.255.0 becomes 0.0.0.255. Router access lists (such as Cisco ACLs) use it to say which address bits may differ. Its 1 bits mark host bits, the 0 bits must match.

Which IPv4 ranges are private?

RFC 1918 reserves 10.0.0.0/8, 172.16.0.0/12 (172.16.0.0 to 172.31.255.255) and 192.168.0.0/16 for private networks. RFC 6598 adds 100.64.0.0/10 for carrier-grade NAT, which is not private in the same sense: it is shared between a provider and its customers. 169.254.0.0/16 is link-local (RFC 3927), 127.0.0.0/8 is loopback, and 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24 are for documentation (RFC 5737).

Can I split a network into subnets of different sizes?

This tool splits into equal subnets: either a number of them (which must be a power of 2, because each extra prefix bit doubles the count) or a prefix length. For unequal sizes (VLSM), split the block once, then split one of the resulting subnets again. The list is capped at 256 subnets; the heading always shows the true total.

How does IPv6 shorten an address?

RFC 5952 sets one canonical form: lowercase hex, no leading zeros in a group, and “::” in place of the longest run of two or more all-zero groups (the first one if there is a tie). 2001:0db8:0000:0000:0000:0000:0000:0001 becomes 2001:db8::1. A single zero group is not replaced: 2001:db8:0:1:1:1:1:1 stays 2001:db8:0:1:1:1:1:1. In 2001:db8:0:0:1:0:0:1 the two runs are equally long, so the first is shortened: 2001:db8::1:0:0:1.

How many /64 networks are in an IPv6 prefix?

A /64 is the standard size of one subnet, so a prefix of length p up to /64 holds 2^(64 − p) of them: a /48 holds 65,536 and a /56 holds 256. A prefix longer than /64, such as a /127 link, is smaller than one /64.

What kinds of IPv6 address does it recognise?

Unique local addresses (fc00::/7, RFC 4193), link-local (fe80::/10), documentation (2001:db8::/32, RFC 3849), multicast (ff00::/8), loopback (::1), the unspecified address (::), IPv4-mapped addresses (::ffff:0:0/96) and global unicast (2000::/3). The table above lists them all.

Is anything I type sent to a server?

No. The address is worked out in your browser with plain arithmetic (BigInt for IPv6) and never leaves your device.