Subnet Mask

A subnet mask is a number that splits an IP address into a network part and a host part, telling a device which addresses are on its local network and which must be reached through a router. For example, the mask 255.255.255.0 means the first three numbers identify the network.

Subnet masks are often written in CIDR notation as a slash and the number of network bits, such as 192.168.1.0/24, which allows 254 usable host addresses. Subnetting divides a large network into smaller ones for better organisation, security and efficiency.

Network:      192.168.1.0/24
Subnet mask:  255.255.255.0
Usable hosts: 192.168.1.1 – 192.168.1.254 (254 hosts)
Broadcast:    192.168.1.255