In plain words
A page to keep at hand: for each network size, each line gives its mask, how many machines it can hold and the gap between two networks of that size. To understand where these figures come from, see subnet masks and CIDR notation.
How to read the table
| Column | What it gives |
|---|---|
| Prefix | The network size in CIDR notation |
| Mask | The same network in classic notation |
| Wildcard | Used in Cisco access lists and OSPF (wildcard mask) |
| Addresses | The total number of addresses in the network |
| Machines | The addresses you can give to machines (total minus the network address and the broadcast address) |
| Block | The gap between two consecutive networks of this size; in brackets, the byte it applies to |
| vs /24 | The size compared with a /24, the usual home network (1/4 = a quarter of a /24) |
The table
| Prefix | Mask | Wildcard | Addresses | Machines | Block | vs /24 |
|---|---|---|---|---|---|---|
| /8 | 255.0.0.0 | 0.255.255.255 | 16,777,216 | 16,777,214 | 1 (1st) | 65,536 × /24 |
| /9 | 255.128.0.0 | 0.127.255.255 | 8,388,608 | 8,388,606 | 128 (2nd) | 32,768 × /24 |
| /10 | 255.192.0.0 | 0.63.255.255 | 4,194,304 | 4,194,302 | 64 (2nd) | 16,384 × /24 |
| /11 | 255.224.0.0 | 0.31.255.255 | 2,097,152 | 2,097,150 | 32 (2nd) | 8,192 × /24 |
| /12 | 255.240.0.0 | 0.15.255.255 | 1,048,576 | 1,048,574 | 16 (2nd) | 4,096 × /24 |
| /13 | 255.248.0.0 | 0.7.255.255 | 524,288 | 524,286 | 8 (2nd) | 2,048 × /24 |
| /14 | 255.252.0.0 | 0.3.255.255 | 262,144 | 262,142 | 4 (2nd) | 1,024 × /24 |
| /15 | 255.254.0.0 | 0.1.255.255 | 131,072 | 131,070 | 2 (2nd) | 512 × /24 |
| /16 | 255.255.0.0 | 0.0.255.255 | 65,536 | 65,534 | 1 (2nd) | 256 × /24 |
| /17 | 255.255.128.0 | 0.0.127.255 | 32,768 | 32,766 | 128 (3rd) | 128 × /24 |
| /18 | 255.255.192.0 | 0.0.63.255 | 16,384 | 16,382 | 64 (3rd) | 64 × /24 |
| /19 | 255.255.224.0 | 0.0.31.255 | 8,192 | 8,190 | 32 (3rd) | 32 × /24 |
| /20 | 255.255.240.0 | 0.0.15.255 | 4,096 | 4,094 | 16 (3rd) | 16 × /24 |
| /21 | 255.255.248.0 | 0.0.7.255 | 2,048 | 2,046 | 8 (3rd) | 8 × /24 |
| /22 | 255.255.252.0 | 0.0.3.255 | 1,024 | 1,022 | 4 (3rd) | 4 × /24 |
| /23 | 255.255.254.0 | 0.0.1.255 | 512 | 510 | 2 (3rd) | 2 × /24 |
| /24 | 255.255.255.0 | 0.0.0.255 | 256 | 254 | 1 (3rd) | 1 × /24 |
| /25 | 255.255.255.128 | 0.0.0.127 | 128 | 126 | 128 (4th) | 1/2 |
| /26 | 255.255.255.192 | 0.0.0.63 | 64 | 62 | 64 (4th) | 1/4 |
| /27 | 255.255.255.224 | 0.0.0.31 | 32 | 30 | 32 (4th) | 1/8 |
| /28 | 255.255.255.240 | 0.0.0.15 | 16 | 14 | 16 (4th) | 1/16 |
| /29 | 255.255.255.248 | 0.0.0.7 | 8 | 6 | 8 (4th) | 1/32 |
| /30 | 255.255.255.252 | 0.0.0.3 | 4 | 2 | 4 (4th) | 1/64 |
| /31 | 255.255.255.254 | 0.0.0.1 | 2 | 2 | 2 (4th) | 1/128 |
| /32 | 255.255.255.255 | 0.0.0.0 | 1 | 1 | 1 (4th) | 1/256 |
For /31 and /32, both addresses (or the single address) are usable: a /31 is used for links between two routers (RFC 3021), a /32 designates a single machine.
The lines to know by heart
| Prefix | Mask | Machines | Where you meet it |
|---|---|---|---|
| /8 | 255.0.0.0 | 16,777,214 | Private range 10.0.0.0/8 |
| /16 | 255.255.0.0 | 65,534 | Private range 192.168.0.0/16 |
| /24 | 255.255.255.0 | 254 | Almost every home network |
| /26 | 255.255.255.192 | 62 | Medium-sized VLAN |
| /27 | 255.255.255.224 | 30 | Small VLAN |
| /30 | 255.255.255.252 | 2 | Link between two routers |
| /32 | 255.255.255.255 | 1 | A single machine (firewall rules) |
Using the block size
The block size lets you find, in your head, the network an address belongs to. Networks of a given size always start on a multiple of the block size.
Example: which network does 192.168.1.130/26 belong to?
- Block size of a /26: 64, on the 4th byte.
- Multiples of 64: 0, 64, 128, 192.
- 130 lies between 128 and 191: the network is
192.168.1.128/26. - The next network starts at 192: so the broadcast address is
192.168.1.191. - Usable machines: from
192.168.1.129to192.168.1.190.
Same method when the block size falls on the 3rd byte: for 10.0.37.5/20, the block size is 16 on the 3rd byte. The multiples of 16 are 0, 16, 32, 48: the network is 10.0.32.0/20, its broadcast 10.0.47.255.
A memory aid for the last byte
Mask values from /25 to /32 follow each other by adding decreasing powers of 2: 128, then 128 + 64 = 192, + 32 = 224, + 16 = 240, + 8 = 248, + 4 = 252, + 2 = 254, + 1 = 255. The block size is simply 256 minus that value.
Key takeaways
- Usable machines: 2^(32 − prefix) − 2, except /31 (2) and /32 (1).
- Block size = 256 − the value of the last non-zero byte of the mask.
- A network always starts on a multiple of its block size.