Match first set of octets in network ID. In other words, tells devices on the network, how many groupings in the IP address to compare, before determining if an IP is local or not.
- An IPv4 subnet mask consists of 32 bits:
- It is a sequence of ones (1) followed by a block of zeros (0).
- The ones indicate bits in the address used for the network prefix and the trailing block of zeros designates that part as being the host identifier. Source
The following example shows the network prefix and host ID for
192.168.5.23 and it’s associated /24 subnet mask 255.255.255.0.
| Dotted Decimal | Binary | |
|---|---|---|
| IP address | 192.168.5.23 | 11000000.10101000.00000101.00010111 |
| Subnet mask | 255.255.255.0 | 11111111.11111111.11111111.00000000 |
| Network ID | 192.168.5.0 | 11000000.10101000.00000101.x |
| (network prefix) | ||
| Host ID | x.x.x.23 | x.x.x.00010111 |
| (rest field) |
The result of the
bitwise AND
operation of IP address and the subnet mask is the network prefix 192.0.2.0.
The host part which is 130, is derived by the bitwise AND operation of the
address and the
one’s complement
of the subnet mask.
- By definition all devices on the same network have the same subnet mask and network ID
Shorthand #
IP address followed by / and number of 1s in the subnet mask.
- 201.23.45.123/24 has subnet mask 255.255.255.0
Class IDs #
Calculating hosts #
2^x - 2 = number of hosts. Where x is number of digits after subnet mask.