A subnet mask is a 32-bit number used in computer networking to divide an IP address into two distinct parts: the network portion and the host portion. The network portion identifies the specific network, while the host portion identifies individual devices (hosts) within that network.
How Subnet Masks Work
• Subnet masks are written in the same format as IP addresses, such as 255.255.255.0.
• In binary, the subnet mask uses a series of 1s to indicate the network part and 0s for the host part. For example, 255.255.255.0 in binary is 11111111.11111111.11111111.00000000, meaning the first three octets (24 bits) specify the network and the last octet (8 bits) specifies the host.
• When a device receives an IP address and subnet mask, it uses a bitwise AND operation to determine which part of the address refers to the network and which to the host.
Purpose and Benefits
• Subnet masks allow networks to be split into smaller sub-networks (subnets), which improves network efficiency, organization, and security.
• They help routers and switches decide if a device is on the same local network or if data needs to be sent to another network, optimizing routing and reducing unnecessary traffic.
• By segmenting a network, subnet masks help contain broadcast traffic within each subnet, reducing congestion and improving performance.
• Subnetting also enhances security by isolating network segments—if one subnet is compromised, the rest remain unaffected.
Example
In a typical home or small office network, an IP address might be 192.168.1.10 with a subnet mask of 255.255.255.0. This means that all devices with addresses from 192.168.1.1 to 192.168.1.254 are on the same network, and the subnet mask helps devices determine whether to communicate directly or route traffic elsewhere.