CIDR / Subnet Calculator

What this checks

• Network and broadcast addresses for the block.
• Subnet mask and wildcard mask in dotted-decimal and binary.
• First and last usable host, total addresses, and usable host count.
• IP class and whether the block falls in RFC 1918 private space.

How to interpret results

• Usable hosts equal total addresses minus network and broadcast for prefixes shorter than /31.
• The wildcard mask is the inverse of the subnet mask and is used in ACLs and OSPF.
• A private result (RFC 1918) means the range is not routable on the public internet.

Common issues

• Entering a host address rather than a network address still works — the tool derives the network from the prefix.
• Only IPv4 is supported in this calculator; use the IPv6 decoder for IPv6 addresses.

CIDR calculator vs subnet calculator

Use the CIDR calculator when you already have one prefix such as 203.0.113.0/24 and want to expand it into network address, broadcast address, host range, mask, wildcard, and host counts. This is the fast reference view for one existing block.

Use the subnet calculator when you need to plan or split a larger network into multiple child networks. That page is optimized for VLSM design, while this page is optimized for interpreting one CIDR block accurately.

Common CIDR examples

Common prefixes map to predictable host counts. A /8 covers a very large block, /16 is often used for broad private networks, /24 is the classic small subnet, and /30 or /31 are common for point-to-point or transit-style addressing.

A /32 represents exactly one IPv4 host. A /31 is a special two-address case that many point-to-point links treat as fully usable under RFC 3021, which is why this calculator does not subtract traditional network and broadcast addresses there.

• /8 = 16,777,216 addresses; /16 = 65,536 addresses; /24 = 256 addresses.
• /30 = 4 addresses, /31 = 2 addresses, /32 = 1 address.

Masks, wildcards, and private ranges

The subnet mask expresses the fixed network bits, while the wildcard mask is the inverse often used in ACLs and routing policy. Seeing both forms together is useful when you move between firewall, DNS, and routing tasks.

Private RFC 1918 ranges such as 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16 are not globally routable. This calculator flags private space so you can quickly distinguish internal networks from public internet allocations.

Privacy note

This calculator runs entirely in the browser. The network you enter is never sent to or stored by the server.

Frequently Asked Questions

What input formats are accepted?

Both CIDR notation (192.168.1.0/24) and address-plus-dotted-mask (192.168.1.0/255.255.255.0 or 192.168.1.0 255.255.255.0).

How are /31 and /32 handled?

A /32 is a single host and a /31 is a 2-address point-to-point link (RFC 3021); for these the calculator reports the addresses as usable rather than subtracting network and broadcast.

Is my input sent to a server?

No. All subnet math runs locally in your browser, so the tool works offline and never transmits the network you enter.

How is this different from the subnet calculator?

The CIDR calculator expands one existing CIDR block and shows its exact range, mask, and host counts. The subnet calculator is better when you need to plan or split a base network into multiple VLSM subnets.