Subnetting

Subnetting is the practice of dividing a larger IP network into smaller subnets. It applies to both IPv4 and IPv6 and uses subnet masks or prefix lengths (CIDR notation) to separate the network portion of an address from the host portion. Subnetting improves address utilization, reduces broadcast domains, and simplifies routing and network management.

In IPv4, a subnet mask indicates which bits belong to the network and which belong to hosts. CIDR notation expresses this as a prefix length after a slash, such as 192.168.1.0/24. A /24 subnet contains 256 addresses. It can be split into two /25 subnets: 192.168.1.0/25 and 192.168.1.128/25, each with 128 addresses.

Key concepts include the network address (the address with all host bits zero) and the broadcast address (all host bits one). Usable host addresses are typically calculated as 2^n − 2 for a subnet with n host bits, though /31 and /32 subnets have special uses. Calculations determine the appropriate mask and address ranges while avoiding overlap with other subnets.

In IPv6, subnetting uses prefix lengths as well, commonly /64 for individual subnets. Subnetting in IPv6 provides a vast address space, simplifies routing tables, and reduces the need for network address translation. A typical example is 2001:db8:abcd:0012::/64, which can be subdivided further as needed.

Reasons to subnet include efficient address space utilization, containment of broadcast domains, improved routing performance, scalability, and enhanced security through network segmentation. Practical subnetting follows a plan: assess host requirements, choose prefix lengths, ensure non-overlapping ranges, configure equipment accordingly, and maintain documentation. Complex networks may use variable length subnet masking (VLSM) to tailor subnets to different needs.