IPv4 subnet calculator

What a subnet mask actually does

Every IPv4 address is 32 bits. A subnet mask (or its shorthand, the CIDR prefix — the /24 in 192.168.1.0/24) splits those 32 bits into a network portion and a host portion. The network portion has to match for two devices to talk directly, without a router in between; the host portion is what's actually available to assign to individual devices. A /24 reserves the first 24 bits for the network, leaving 8 bits — 256 possible values — for hosts. Subtract the network address and broadcast address (neither is assignable to a device) and you get 254 usable hosts, which is exactly what this calculator shows for a default /24.

Reading the bit-level view

The binary breakdown at the bottom of the calculator is the actual mechanism, not just a visualization. Every bit set to 1 in the mask is network; every bit set to 0 is host. Bitwise AND the IP address against the mask and you get the network address — the first address in the range, never assignable. Flip every host bit to 1 and you get the broadcast address — the last address, also never assignable. Everything between those two is usable. This is worth understanding once, by eye, because it's the same operation your router, firewall rules, and DHCP scope are all doing under the hood every time they decide whether two addresses are "on the same network."

Common prefix lengths, at a glance

The pattern: each step down in prefix length (say /25 to /24) doubles the address space. Every step up does the opposite — halves it. Getting comfortable with that doubling relationship is most of what "understanding subnetting" actually means in practice.

Special cases this calculator handles

• /31 (RFC 3021) — normally too small to have any usable hosts (2 total addresses, both would be network+broadcast), but RFC 3021 explicitly permits /31 as a point-to-point link where both addresses are usable. This calculator applies that exception rather than showing zero usable hosts.
• /32 — a single host route, not a subnet in the traditional sense. One address, itself, commonly used for loopbacks and specific static routes.
• Range type detection — the calculator flags RFC 1918 private ranges (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), loopback (127.0.0.0/8), link-local (169.254.0.0/16), and other reserved ranges automatically, so you can tell at a glance whether an address is routable on the public internet.

Worked example

You're given 10.20.6.130/26 and need to know what network it belongs to. A /26 mask is 255.255.255.192, meaning the last octet splits into blocks of 64 (256÷4, since a /26 leaves 2 host bits worth of subnetting — four /26 blocks per /24). 130 falls in the third block: 0–63, 64–127, 128–191, 192–255. So the network address is 10.20.6.128, broadcast is 10.20.6.191, and usable hosts run from 10.20.6.129 to 10.20.6.190 — 62 addresses. Paste 10.20.6.130/26 into the calculator above to confirm.