About CIDRGen

What this is

A subnet calculator that runs in your browser. Nothing you type is uploaded.

For an IPv4 prefix it reports the network and broadcast addresses, the usable range, the netmask and wildcard mask, total and usable host counts, and whether the block is private, shared (CGNAT), link-local, loopback, documentation, multicast, reserved or public. For an IPv6 prefix it reports the first and last address and the address count; IPv6 has no broadcast and no "minus two".

Rules it applies

• /31 and /32 are special: a /31 is a point-to-point link with two usable addresses and no broadcast (RFC 3021), and a /32 is one host. Everything shorter reserves the network and broadcast addresses.
• An address with host bits set (10.0.1.100/25) is not a network address. The tool shows the block it really names (10.0.1.0/25) and warns, because cloud consoles often accept the string and silently use the network address.
• Octets with leading zeros (010.0.0.1) are rejected: some stacks read them as octal.
• The special-range table is the IANA IPv4 special-purpose list for the common blocks. A block that straddles two ranges is classified by its network address.

Companion tools

• Subnet splitter: cut a block into equal subnets.
• CIDR overlap checker: find overlaps and get the minimal aggregate of a list.
• Netmask converter: prefix, netmask and wildcard in any direction.

What it will not do

It does not know your cloud provider's reserved addresses (AWS, GCP and Azure each hold back extra ones in every subnet), and it does not split IPv6 blocks.