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Subnetting practice
An address and a prefix: work out its network and broadcast addresses, first and last host, how many hosts it holds and its mask. Check your answers and see the working, as many times as you like.
The method: find the block
Every question is solved the same way, in the one octet where the prefix ends. The octets before it are copied from the address; the octets after it are 0 in the network and 255 in the broadcast.
- The octet. A prefix of 8 or less ends in the first octet, 9 to 16 in the second, 17 to 24 in the third, 25 to 32 in the fourth (a prefix of exactly 8, 16 or 24 leaves the whole next octet to hosts).
- The mask value in that octet comes from how many of its bits are network bits, in the table below: /20 has 4 network bits in the third octet, so its mask is 255.255.240.0.
- The block size is 256 less that mask value. Subnets start at every multiple of the block in that octet.
- The network is the multiple of the block at or below the address's value in that octet; the broadcast is the next multiple less one.
- The hosts lie between them, from the network plus one to the broadcast less one, and there are 2host bits − 2 of them.
| Network bits | Mask | Block |
|---|---|---|
| 1 | 128 | 128 |
| 2 | 192 | 64 |
| 3 | 224 | 32 |
| 4 | 240 | 16 |
| 5 | 248 | 8 |
| 6 | 252 | 4 |
| 7 | 254 | 2 |
| 8 | 255 | 1 |
Worked examples
192.168.10.77/27
- Subnet mask. /27 is 27 ones then 5 zeros: 255.255.255.224. The prefix ends in the fourth octet, so that octet is where the work is.
- Block size. 256 − 224 = 32: subnets start at every multiple of 32 in the fourth octet: 0, 32, 64, 96, ….
- Network. 77 falls in the block 64 to 95, so the network is its start: 192.168.10.64.
- Broadcast. The block's end, 95: 192.168.10.95.
- First and last host. One after the network and one before the broadcast: 192.168.10.65 to 192.168.10.94.
- Usable hosts. 2⁵ − 2 = 30: two to the power of the 5 host bits, less the network and broadcast addresses.
- Network address
- 192.168.10.64
- Broadcast address
- 192.168.10.95
- First host
- 192.168.10.65
- Last host
- 192.168.10.94
- Usable hosts
- 30
- Subnet mask
- 255.255.255.224
172.16.77.9/20
- Subnet mask. /20 is 20 ones then 12 zeros: 255.255.240.0. The prefix ends in the third octet, so that octet is where the work is.
- Block size. 256 − 240 = 16: subnets start at every multiple of 16 in the third octet: 0, 16, 32, 48, ….
- Network. 77 falls in the block 64 to 79, so the network is its start, with 0 in every octet after it: 172.16.64.0.
- Broadcast. The block's end, 79, with 255 in every octet after it: 172.16.79.255.
- First and last host. One after the network and one before the broadcast: 172.16.64.1 to 172.16.79.254.
- Usable hosts. 2¹² − 2 = 4,094: two to the power of the 12 host bits, less the network and broadcast addresses.
- Network address
- 172.16.64.0
- Broadcast address
- 172.16.79.255
- First host
- 172.16.64.1
- Last host
- 172.16.79.254
- Usable hosts
- 4,094
- Subnet mask
- 255.255.240.0
10.45.130.200/11
- Subnet mask. /11 is 11 ones then 21 zeros: 255.224.0.0. The prefix ends in the second octet, so that octet is where the work is.
- Block size. 256 − 224 = 32: subnets start at every multiple of 32 in the second octet: 0, 32, 64, 96, ….
- Network. 45 falls in the block 32 to 63, so the network is its start, with 0 in every octet after it: 10.32.0.0.
- Broadcast. The block's end, 63, with 255 in every octet after it: 10.63.255.255.
- First and last host. One after the network and one before the broadcast: 10.32.0.1 to 10.63.255.254.
- Usable hosts. 2²¹ − 2 = 2,097,150: two to the power of the 21 host bits, less the network and broadcast addresses.
- Network address
- 10.32.0.0
- Broadcast address
- 10.63.255.255
- First host
- 10.32.0.1
- Last host
- 10.63.255.254
- Usable hosts
- 2,097,150
- Subnet mask
- 255.224.0.0
To check any address and prefix of your own, with its binary form and its split into smaller subnets, use the subnet calculator.