Subnet calculator

Type an address and mask in any common form: 192.168.1.10/24, 192.168.1.10 255.255.255.0, or the address and a host count you need. The result updates as you type.

The network address is the first address of the block, with all host bits set to 0 (192.168.1.0 for /24); the broadcast address is the last, with all host bits set to 1 (192.168.1.255). Neither can be assigned to a device on a normal LAN.

Everything between network and broadcast: 2^(32 − prefix) − 2 addresses, so a /24 has 254 usable hosts, a /30 has 2 and a /31 (point-to-point links, RFC 3021) uses both addresses. A /32 is a single host.

The mask has the prefix-length leading bits set (/24 = 255.255.255.0); the wildcard mask is its inverse (0.0.0.255) and is what access lists on Cisco equipment expect. Both are shown in dotted-decimal and binary.

Enter the block you have and the sizes you need, and the calculator divides it into the smallest subnets that fit, largest first, with each subnet's range. This is variable-length subnet masking, the normal way to carve an office or data center block.

Enter an IPv6 address with a prefix, for example 2001:db8:abcd::/48, to get the first and last address of the block, the compressed and expanded forms, the number of /64 networks it contains and the reverse DNS zone name. IPv6 has no broadcast address and host counts are astronomical, so the useful question is almost always how many /64s do I get — a /48 gives 65,536 of them, a /56 gives 256, the typical home allocation.

IPv6: a /64 is one LAN; a /56 holds 256 of them; a /48 holds 65,536; a /32 is a typical ISP allocation.

The number after the slash is how many leading bits identify the network; the rest identify hosts within it. /24 leaves 8 host bits (256 addresses), /16 leaves 16 (65,536), /8 leaves 24 (16.7 million). These three align with the old class C, B and A boundaries, which is why they are still the most quoted; any prefix length from /0 to /32 is valid. See CIDR notation explained.

An IPv4 address is 32 bits. The subnet mask says how many of those bits, from the left, name the network; a device compares the destination address with its own under the mask to decide whether to send directly (same network) or to the router (different network). Subnetting takes a block and makes the network part longer, creating several smaller networks — one per office floor, per VLAN, per customer. The guides what is a subnet mask and IP address classes cover the background; the CIDR to IP range converter does the reverse of this calculator.