How subnetting works
An IPv4 address is 32 bits. The prefix length after the slash says how many of those bits identify the network; the rest identify the host within it. A /26 fixes the first 26 bits, leaving 6 host bits, so the block holds 26 = 64 addresses.
Because the split happens on bit boundaries, blocks always have a power-of-two size and always start on a multiple of that size. This is why 192.168.1.130/26 belongs to the subnet starting at 192.168.1.128 and not at 192.168.1.130 — 130 is not a multiple of 64.
Worked example: 192.168.1.130/26
- Mask — 26 network bits gives 255.255.255.192.
- Network — 130 rounded down to a multiple of 64 is 128, so the subnet is 192.168.1.128/26.
- Broadcast — the last address in the block: 192.168.1.128 + 64 − 1 = 192.168.1.191.
- Usable range — 192.168.1.129 to 192.168.1.190, which is 62 hosts.
IPv4 subnet mask reference
Every prefix length from /8 to /32, with its mask and host count.
| CIDR | Subnet mask | Addresses | Usable hosts |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,216 | 16,777,214 |
| /9 | 255.128.0.0 | 8,388,608 | 8,388,606 |
| /10 | 255.192.0.0 | 4,194,304 | 4,194,302 |
| /11 | 255.224.0.0 | 2,097,152 | 2,097,150 |
| /12 | 255.240.0.0 | 1,048,576 | 1,048,574 |
| /13 | 255.248.0.0 | 524,288 | 524,286 |
| /14 | 255.252.0.0 | 262,144 | 262,142 |
| /15 | 255.254.0.0 | 131,072 | 131,070 |
| /16 | 255.255.0.0 | 65,536 | 65,534 |
| /17 | 255.255.128.0 | 32,768 | 32,766 |
| /18 | 255.255.192.0 | 16,384 | 16,382 |
| /19 | 255.255.224.0 | 8,192 | 8,190 |
| /20 | 255.255.240.0 | 4,096 | 4,094 |
| /21 | 255.255.248.0 | 2,048 | 2,046 |
| /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 |
Frequently asked
Why does a /24 have 254 usable hosts and not 256?
The first address in a block is the network address, which names the subnet itself, and the last is the broadcast address, which reaches every host on it. Neither can be assigned to a machine, so a /24 holds 256 addresses but only 254 usable ones.
Why does this calculator say a /31 has 2 usable hosts?
RFC 3021 allows a /31 on point-to-point links, where there is no need for a broadcast address because there is only one other device. Both addresses are usable. Most older calculators still report 0, which is wrong for the links /31 exists to serve.
What is a wildcard mask?
A wildcard mask is the bitwise inverse of the subnet mask: where the subnet mask has ones, the wildcard has zeros. Cisco access control lists and OSPF use it to say which bits must match, so a /24 subnet mask of 255.255.255.0 becomes a wildcard of 0.0.0.255.
Does the tool accept an address that is not the network address?
Yes. Give it 192.168.1.130/26 and it works out that the address sits in the 192.168.1.128/26 subnet, then reports that subnet's values. You do not have to know the network address in advance — that is usually what you are trying to find.