A subnet (subnetwork) is a logical partition of an IP network created by using a subnet mask to divide a larger network into smaller sections. Each subnet operates as its own distinct network segment, allowing devices within it to communicate directly without routing through a gateway.
When you set up a home or office network, your router typically creates a single subnet. For example, a home network might use the subnet 192.168.1.0/24, which provides 254 usable addresses (192.168.1.1 through 192.168.1.254). The subnet mask (255.255.255.0 in this case) determines which portion of an IP address identifies the network and which portion identifies individual devices.
Larger organisations use subnetting to improve network efficiency and security. By breaking a large network into multiple subnets, administrators can:
- Reduce broadcast traffic that clogs the network
- Isolate departments or functions for security purposes
- Manage IP addresses more effectively
- Improve network performance by limiting collision domains
Understanding subnets matters if you're troubleshooting connectivity issues, setting up a home lab, or configuring network equipment. If two devices don't communicate even when connected to the same router, they may be on different subnets. When buying networked devices or setting up home automation, knowing your subnet range helps you identify whether new devices can access your network properly.
Subnetting uses CIDR notation (Classless Inter-Domain Routing) to express subnet sizes. The notation /24 means the first 24 bits identify the network, leaving 8 bits for device addresses. Smaller numbers like /16 create larger subnets with more addresses; larger numbers like /30 create tiny subnets for point-to-point links.