CIDR / subnet calculator

A CIDR calculator takes an IPv4 address written as a CIDR block, like 192.168.1.0/24, or as an IP address and a netmask, like 192.168.1.0 255.255.255.0, and works out everything about that subnet. It shows the network address, the broadcast address, the first and last usable host, how many hosts the subnet can hold, the netmask and wildcard mask, and a binary view of the address with the network and host portions marked. It also flags what kind of address you entered, such as private, loopback, or public.

Is my data uploaded anywhere?

No. Everything runs in your browser, and the addresses and subnets you type never leave your machine, so it’s fine to work out internal network ranges, VPN configs, or firewall rules without sending anything to a server.

How are /31 and /32 subnets handled?

Most subnets reserve their first address for the network itself and their last address for the broadcast, so a /24 with 256 total addresses only has 254 usable for hosts. Two prefix lengths are exceptions. A /32 describes a single address with no room for a separate network or broadcast address, so the calculator treats that one address as both the network and the only usable host. A /31 has just two addresses total and normally neither would be usable under the standard rule, but RFC 3021 defines /31 for point-to-point links and treats both addresses as usable hosts, which is how this calculator handles it too.

What do the address classifications mean?

The calculator checks the network address against the ranges RFC 1918 and related standards set aside for special use, and labels it accordingly: private (the familiar 10.x, 172.16-31.x, and 192.168.x ranges used inside home and office networks), loopback (127.x, which routes back to the same machine), link-local (169.254.x, assigned automatically when no DHCP server responds), CGNAT (100.64-127.x, used by ISPs to share a single public address across many customers), multicast (224-239.x, for one-to-many traffic), reserved (addresses set aside for other special purposes), or public (everything else, routable on the open internet). This is meant as a quick sanity check, not a substitute for looking up the exact standard when it matters.

Use it to check whether two hosts fall in the same subnet, work out how many usable addresses a /27 or /29 gives you, convert between CIDR notation and a dotted netmask, or verify a private range. See RFC 4632 for the full CIDR specification. IPv6 support may be added later.