IPv4 Subnet & CIDR Calculator

An IPv4 address is a 32-bit number, usually written as four decimal octets such as 192.168.1.10. A subnet mask splits those 32 bits into a network portion and a host portion. CIDR (Classless Inter-Domain Routing) notation expresses the mask as a prefix length: /24 means the first 24 bits identify the network, which is the same as the netmask 255.255.255.0.

For any subnet, the address with all host bits set to 0 is the network address and the address with all host bits set to 1 is the broadcast address. Neither can normally be assigned to a device, so a subnet with n host bits has 2ⁿ total addresses and 2ⁿ − 2 usable hosts. Two exceptions are handled here: a /31 is used for point-to-point links (RFC 3021) where both addresses are usable, and a /32 describes a single host route.

The wildcard mask is the bitwise inverse of the netmask and is used in Cisco ACLs and OSPF configuration. The subnet splitter divides a network into equal, smaller subnets — for example, splitting a /24 into four subnets produces four /26 networks of 62 usable hosts each — which is the basis of planning VLANs, cloud VPC subnets and firewall zones.

The calculator also identifies special-purpose ranges: private RFC 1918 space (10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16), carrier-grade NAT (100.64.0.0/10), loopback (127.0.0.0/8), link-local (169.254.0.0/16), documentation and multicast blocks. Legacy class (A–E) is shown for reference, although modern routing is classless. All calculations happen locally in your browser.

Related tools: find the public address your network uses on the internet with What Is My IP, and convert addresses or masks between decimal, hex and binary with the number base converter.