How to calculate a subnet
- Type an address with its prefix, like 10.20.30.40/22 or 2001:db8::/48, into IP address / prefix. For IPv4 you can also give the subnet mask after a space: 10.20.30.40 255.255.252.0.
- Read the network, the number of usable hosts and the address type at the top, then the full breakdown below.
- Click any value, such as the subnet mask or the first usable host, to copy it.
Common subnet sizes
| Prefix | Subnet mask | Addresses | Usable hosts |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /20 | 255.255.240.0 | 4,096 | 4,094 |
| /22 | 255.255.252.0 | 1,024 | 1,022 |
| /23 | 255.255.254.0 | 512 | 510 |
| /24 | 255.255.255.0 | 256 | 254 |
| /25 | 255.255.255.128 | 128 | 126 |
| /26 | 255.255.255.192 | 64 | 62 |
| /27 | 255.255.255.224 | 32 | 30 |
| /28 | 255.255.255.240 | 16 | 14 |
| /29 | 255.255.255.248 | 8 | 6 |
| /30 | 255.255.255.252 | 4 | 2 |
| /31 | 255.255.255.254 | 2 | 2 |
| /32 | 255.255.255.255 | 1 | 1 |
Each bit you take off the prefix doubles the size of the subnet, so a /23 holds two /24s. To pick a size, find the smallest subnet whose usable hosts cover your devices plus room to grow.
Private and special address ranges
The calculator labels the address you enter with its type:
- Private (RFC 1918): 10.0.0.0/8, 172.16.0.0/12 and 192.168.0.0/16. Used inside home and office networks and never routed on the internet.
- Loopback: 127.0.0.0/8. Traffic to these addresses never leaves the machine.
- Link-local: 169.254.0.0/16. A device picks one of these when it can’t get an address from DHCP, which is a common sign of a network problem.
- Shared address space: 100.64.0.0/10, used by internet providers for carrier-grade NAT.
- Multicast: 224.0.0.0 to 239.255.255.255, for one-to-many traffic.
- Public: everything else outside the reserved ranges, routed on the internet.