Subnet Calculator
IPv4 & IPv6 CIDR Calculator

CIDR (Classless Inter-Domain Routing) notation writes a network as an address followed by a slash and a prefix length, such as 192.168.1.0/24. The prefix length is the number of leading bits that identify the network; the remaining bits identify hosts. A /24 has 24 network bits and 8 host bits, which is the same as the netmask 255.255.255.0.

For IPv4, raise 2 to the number of host bits and subtract 2 for the network and broadcast addresses: a /24 has 2^8 - 2 = 254 usable hosts and a /26 has 2^6 - 2 = 62. The exceptions are /31, which has two usable addresses on a point-to-point link, and /32, which is a single host. IPv6 has no broadcast address, so every address in the prefix counts.

RFC 3021 allows /31 networks on point-to-point links. A link between two routers only ever has two endpoints and never needs a broadcast address, so both addresses are assigned to the two interfaces. That saves two addresses per link compared with a /30, and every major router operating system supports it.

A subnet mask has 1 bits for the network part (255.255.255.0 for a /24). A wildcard mask is its exact inverse, with 1 bits for the host part (0.0.0.255 for a /24). Cisco access control lists and OSPF network statements use wildcard masks, while interface and DHCP configuration use subnet masks. The calculator shows both.

Yes. Type the address followed by a space or a slash and the dotted netmask, for example 172.16.5.4 255.255.240.0 or 172.16.5.4/255.255.240.0. The netmask must be contiguous - all the 1 bits before the 0 bits. If you paste a wildcard mask such as 0.0.0.255 by mistake, the calculator tells you the matching netmask.

If you type a host address such as 192.168.1.130/26, the host bits are not zero, so it is not the network address itself. The calculator clears the host bits to find the network the host belongs to (192.168.1.128/26) and shows a note. Your typed address stays in the input box and is still used for the address type, integer, and binary rows.

Yes. Enter any IPv6 address with a prefix from /0 to /128, including compressed forms like 2001:db8::/32 and IPv4-mapped addresses. All math uses 128-bit integers, so address counts are exact even for a /0. IPv6 has no broadcast address, and /64 is the standard size for a LAN because SLAAC autoconfiguration needs 64 host bits.

AWS reserves five addresses in every subnet: the network address, the next three (VPC router, DNS, and future use), and the last address. A /24 subnet therefore has 256 - 5 = 251 usable addresses in AWS, not 254. Azure also reserves five per subnet. The calculator shows the standard count; subtract your cloud provider's extra reserved addresses from it.

Use the Subnet Split Planner. Pick a new, longer prefix (a /24 split at /26 gives four subnets), the number of subnets you need (rounded up to a power of two), or the hosts each subnet must hold. The planner lists every resulting subnet with its host range and broadcast address, and exports the list as CSV.

No. Every calculation runs in your browser with JavaScript. The addresses you enter are never sent to a server, logged, or stored - which makes the calculator safe to use with internal network plans and production IP ranges.