CIDR / subnet calculator.

What is CIDR?

CIDR — Classless Inter-Domain Routing — is the notation we use to describe a contiguous range of IP addresses. 10.0.0.0/24 means "the 256 addresses starting at 10.0.0.0," because the /24 says the first 24 bits are the network and the remaining 8 are host space.

How to read the results

• Network address — the first address in the block. By convention, this isn't assignable to a host on classful IPv4 segments.
• Broadcast address (IPv4 only) — the last address. Also not assignable to a host; sending to it reaches every device on the segment.
• First/Last host — the range you can actually hand out to devices.
• Netmask — the same prefix length expressed as a dotted-quad mask. /24 is 255.255.255.0.
• Wildcard mask — the bitwise inverse of the netmask. Cisco ACLs use it.
• Usable hosts — total addresses minus 2 (network + broadcast) for IPv4 on /30 and shorter. /31 point-to-point links use both endpoints (RFC 3021). /32 is a single host. IPv6 has no broadcast, so every address in a prefix is usable.

Common subnet sizes