IP Subnet Calculator

It is the CIDR prefix — the number of bits that identify the network. /24 means the first 24 bits (the first three octets) are the network, leaving 8 bits for hosts, which is 256 addresses and a 255.255.255.0 mask.

The network address is the first address in the block (all host bits 0) and names the subnet itself. The broadcast address is the last (all host bits 1) and reaches every host at once. Neither is assigned to a device, which is why they are excluded from the usable count.

For most prefixes it is 2^(32−prefix) − 2, because the network and broadcast addresses are reserved. A /24 has 254, a /25 has 126, a /26 has 62. A /31 has 2 (point-to-point) and a /32 has 1.

It is the prefix written as a dotted-quad, with network bits set to 1. /24 is 255.255.255.0, /26 is 255.255.255.192. ANDing an address with the mask yields its network address, which is how a device decides whether a destination is local or must go through the gateway.

It is the subnet mask inverted (0s where the mask has 1s). Cisco access control lists and some routing tools take the wildcard form instead of the mask, so a /24 is written with wildcard 0.0.0.255.

Yes. Type something like 10.0.0.0/16 into the address field and the tool splits off the /16 and sets the prefix selector for you. Or set the prefix separately with the dropdown.

The RFC 1918 ranges: 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. These are not routed on the public internet and are used inside home and office networks. The tool labels each address as private or public.

Not yet — this tool covers IPv4 only, which is where subnet questions usually come up. IPv6 uses the same prefix idea but 128-bit addresses and different conventions.

No. The calculation is bit arithmetic done entirely in your browser; the address you enter is never uploaded and analytics never receives it.