Simple definition….
An IPv4 subnet mask identifies which bits of an IP address form the network prefix and which bits identify hosts on that network.
Technical definition….
For example, 255.255.255.0 corresponds to a /24 IPv4 prefix: the first 24 bits represent the network portion.
What’s the point?… ?
Set the size of a subnet, calculate the ranges you can use and organize a of address plan….
Concrete example…
For 192.168.1.25/24, the network address is 192.168.1.0 and the broadcast address is 192.168.1.255. In a typical subnet, assignable host addresses run from 192.168.1.1 to 192.168.1.254.
Common problems….
- Different masks on devices of the same network….
- Overlapping sub-networks…
- Range too small for number of, equipment….
- Confusion between mask and bridge….
Related terms….
Note: Exact settings and behavior depend on the product, software version and network environment.