Subnet and CIDR calculator
Enter an IPv4 block as CIDR (10.0.0.0/24) or as an address and netmask. Everything is calculated in your browser — no network request is made.
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Divide into equal subnets
Three things that catch people out.
Why /31 and /32 differ
A /31 is a point-to-point link (RFC 3021) and a /32 is a single host — neither reserves network and broadcast addresses, so both addresses are usable. Many calculators get this wrong and report −2 hosts.
Wildcard masks
The wildcard is the inverse of the netmask. Cisco ACLs and some firewall syntaxes want it instead of the mask, and getting the two confused is a common source of rules that silently match nothing.
Leave room to grow
Sizing a subnet exactly to today’s host count means renumbering later. Pick the next prefix up unless address space is genuinely scarce.
Check something else
Found something you’d rather not fix yourself?
Configuring DNS, TLS and email authentication properly — without breaking live traffic — is part of our cybersecurity and managed services work.