FLSM

Fixed-Length Subnet Mask (FLSM) is a network design approach in IP addressing where all subnets derived from a common network share the same subnet mask, producing subnets of equal size. This simplicity can ease planning, documentation, and basic routing.

In an FLSM design, the major network is divided into multiple equal-sized subnets by applying a single mask to all subnets. For example, a 192.168.0.0/24 network could be subdivided into four equal subnets using a /26 mask: 192.168.0.0/26, 192.168.0.64/26, 192.168.0.128/26, and 192.168.0.192/26. Each /26 subnet provides up to 62 usable host addresses (64 total minus network and broadcast).

Advantages of FLSM include predictable subnet sizes, straightforward address planning, and simple routing and access-control lists, which can be beneficial in small or tightly controlled environments. The uniformity also makes it easy to estimate bandwidth and capacity across subnets and can simplify basic subnetting calculations.

Limitations include inefficiency in address usage when subnets do not require the same number of hosts, leading to wasted addresses. FLSM is less flexible than Variable-Length Subnet Masking (VLSM), which allows subnets of different sizes to fit actual needs more closely and can reduce the total address space required. In modern networks, VLSM and CIDR are more common for scalable addressing, though FLSM may still be used in legacy setups or where uniform subnet sizes are specifically desired. In IPv6, subnetting often follows fixed-size conventions as well, but with a different address space and scaling considerations.