Subnetting is a fundamental networking skill that every administrator must master. By dividing a network into smaller subnets, you gain better control over broadcast domains, improve security through segmentation, and make more efficient use of your IP address space.
Calculating Subnets and Host Ranges
CIDR (Classless Inter-Domain Routing) notation expresses a network address and its subnet mask in a compact format like 192.168.1.0/24. The number after the slash indicates how many bits are used for the network portion, with the remaining bits available for host addresses. A /24 network provides 254 usable host addresses, while a /27 provides 30.
When designing your subnet scheme, plan for growth by allocating more address space than currently needed. It is far easier to leave room in your addressing plan than to renumber an entire subnet later. Use consistent subnet sizes for similar functions, such as /24 for server VLANs and /22 for user VLANs.
Private address ranges defined in RFC 1918 (10.0.0.0/8, 172.16.0.0/12, and 192.168.0.0/16) should be used for internal networks. Document your subnet allocations in an IP Address Management (IPAM) tool or at minimum a well-maintained spreadsheet. This documentation becomes critical during troubleshooting and is essential for onboarding new network team members.