[Cisco IOS Software Basics] What is the purpose of a subnet mask?

A subnet mask is a 32-bit number used in IP networking to divide an IP address into network and

host portions. This division is crucial for routing traffic within and between networks. The subnet

mask works in tandem with the IP address to identify which part of the address refers to the network

and which part refers to the host. This distinction allows for efficient IP address allocation and

routing.​

For example, in the IP address 192.168.1.10 with a subnet mask of 255.255.255.0, the first three

octets (192.168.1) represent the network portion, while the last octet (10) identifies the specific host

within that network.​

Reference: Supporting Cisco Devices for Field Technicians (FLDTEC) – Cisco IOS Software Basics