Subnet / CIDR Calculator

What CIDR notation means

An address like 192.168.1.10/24 pairs a 32-bit IPv4 address with a prefix length — the number of leading bits that identify the network. The remaining bits identify hosts within it. A /24 fixes the first 24 bits (three octets) as the network, leaving 8 bits — 256 addresses, 254 of them usable — for hosts.

The five numbers that fall out of it

• Network address — host bits all zero; names the subnet itself.
• Broadcast address — host bits all one; reaches every host at once.
• First / last host — the usable range between them.
• Subnet mask — the prefix written as dotted decimal (/24 = 255.255.255.0).
• Wildcard mask — the mask inverted, used by ACLs and OSPF.

The two addresses you lose (network and broadcast) are why a /24 gives 254 usable hosts, not 256.

Prefix cheat sheet

Private address ranges (RFC 1918)

These never appear on the public internet: 10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16. The calculator flags which one an address belongs to, plus loopback (127/8) and link-local (169.254/16).

Frequently asked questions

Why does /30 give only 2 usable hosts?

Four addresses, minus the network and broadcast, leaves two — exactly enough for a point-to-point link between two routers, which is its main use.

What's special about /31?

RFC 3021 allows /31 on point-to-point links with no network or broadcast address, so both addresses are usable — a way to save address space on router-to-router links.

Can I type the mask instead of the prefix?

Yes — put a dotted mask like 255.255.255.192 in the prefix field, or append /26 right onto the address. Either works.

What about IPv6?

This calculator is IPv4. IPv6 uses the same prefix idea but 128-bit addresses and hexadecimal notation — a separate tool.