IP Class Calculator

Identify the class of an IPv4 address

Before CIDR, IPv4 addresses were divided into classes based on the value of the first octet, each with a fixed default subnet mask. This calculator identifies the class of any address: class A covers first octets from 0 to 127 with a /8 mask, class B covers 128 to 191 with a /16 mask, class C covers 192 to 223 with a /24 mask, class D from 224 to 239 is reserved for multicast, and class E from 240 to 255 is reserved and experimental. It also reports whether the address is a private RFC 1918 address, a loopback address, or public.

The result shows the first octet value so you can see exactly why the address falls into its class.

Classful addressing today

Although the internet moved to classless CIDR routing long ago, address classes are still taught because they explain the historical structure of the address space and appear constantly in certification exams. The default masks also remain useful shorthand, and the private ranges defined within the classes are very much in active use on home and office networks.

Private addresses, the 10, 172.16 to 172.31, and 192.168 ranges, are reserved for internal use and are not routed on the public internet, which is why recognising them matters for network design. All calculation happens locally in your browser.

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