Subnet and CIDR calculator
Normalize a prefix, inspect its address boundaries and copy the result. Calculating a block does not perform an IP lookup.
203.12.0.0/16
Normalized IPv4 network
16 network bits16 host bits
- First address
- 203.12.0.0
- Last address
- 203.12.255.255
- Total addresses
- 65,536
- Prefix length
- /16
IPv4 host range
- Subnet mask
- 255.255.0.0
- Wildcard mask
- 0.0.255.255
- Usable addresses
- 65,534
- First usable
- 203.12.0.1
- Last usable
- 203.12.255.254
The usable count excludes the network and broadcast addresses. It describes address space, not reachable hosts.
Reading the calculation
The prefix fixes the network bits. The remaining n bits give 2^n addresses. This is the size of the block, not an allocation, announced route or count of active devices.
IPv4 /31 point-to-point links use both addresses; a /32 identifies one address. IPv6 has no broadcast address. Documentation blocks such as 192.0.2.0/24 and 2001:db8::/32 do not imply public routability.