CIDR Calculator with IP Range to CIDR and IPv6

How CIDR notation and the subnet calculator work

CIDR (Classless Inter-Domain Routing) writes a network as an address plus a prefix length, such as 192.168.1.0/24. The prefix says how many leading bits identify the network; the rest number the hosts. A /24 leaves 8 host bits, so it spans 2^8 = 256 addresses, from 192.168.1.0 to 192.168.1.255. Its subnet mask is the prefix written as an address, 255.255.255.0, and the wildcard mask used by Cisco ACLs and OSPF is the inverse, 0.0.0.255.

Type any IPv4 address with a prefix (10.20.30.40/20), with a dotted mask (10.20.30.40 255.255.240.0) or on its own, which is treated as /32. The calculator ANDs the address with the mask to find the network address, sets all host bits to find the broadcast address, and reports the first and last usable hosts, the usable host count, the total address count, the class and whether the range is private (RFC 1918), carrier-grade NAT, loopback, link-local, documentation or public. If you enter a host address such as 172.16.5.10/20 it tells you which network that host belongs to, 172.16.0.0/20. The binary view lines up the address, mask and network bit by bit, which is the quickest way to see why a /20 boundary falls where it does.

Two prefixes break the usual "minus two" rule, and the calculator handles both. A /31 has only two addresses, and RFC 3021 allows both to be used as hosts on a point-to-point link, so it shows 2 usable hosts and no broadcast address. A /32 is a single host route, common for loopbacks and firewall rules, with one usable address. Every other prefix reserves the network and broadcast addresses, giving 2^(32 - prefix) - 2 usable hosts.

IP range to CIDR, and calculating CIDR from a subnet mask

Firewalls, cloud security groups and allow-lists usually want CIDR blocks, but vendors often publish ranges as start and end addresses. The IP range to CIDR panel converts any inclusive range into the smallest list of blocks that covers it exactly, with no addresses added or missed. 10.0.0.0 to 10.0.0.255 becomes the single block 10.0.0.0/24, while 10.0.0.0 to 10.0.1.127 needs two blocks, 10.0.0.0/24 and 10.0.1.0/25. The algorithm takes the largest block that starts on an aligned boundary and does not pass the end address, then repeats from the next address.

To calculate CIDR from a subnet mask, type the mask into the mask to prefix box: 255.255.255.192 is /26 because it has 26 one-bits. A mask with gaps, such as 255.0.255.0, is rejected because it cannot be expressed as a prefix. The reverse box turns a prefix into its mask and wildcard, and the cheat sheet lists every prefix from /0 to /32 with its mask, address count and usable hosts.

IPv6 prefixes

Enter an IPv6 address with a prefix, such as 2001:db8:abcd::/48, and the calculator returns the network, the first and last addresses in both compressed (RFC 5952) and fully expanded form, and the address count as a power of two. IPv6 has no broadcast address and subnets are normally /64, so host-count arithmetic matters less than knowing where a prefix starts and ends. All calculations run locally in your browser using JavaScript numbers for IPv4 and BigInt for 128-bit IPv6 addresses.