Why use VLSM instead of fixed-size subnets?

Fixed-size subnets (FLSM) waste addresses — a /24 for a point-to-point link wastes 252 addresses. VLSM allocates right-sized subnets: /30 for point-to-point, /27 for a small office, /24 for a large department. This can save 50%+ of address space.

How do I align subnets?

A /26 must start at an address divisible by 64 (the block size). 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/26 are valid. 192.168.1.32/26 is invalid — it crosses the 192.168.1.64 boundary. Calculate block size as 2^(32-prefix) and align to multiples of it.

TASK

Divide a network into smaller subnets (VLSM)

Variable Length Subnet Masking (VLSM) divides a network into subnets of different sizes, allocating address space efficiently based on need.

1

VLSM allows subnets of different sizes within the same parent network. For example, a /24 can be divided into: one /25 (126 hosts), one /26 (62 hosts), two /27 (30 hosts each), and two /28 (14 hosts each). Allocate the largest subnets first to avoid fragmentation. Each subnet must be aligned to its size boundary.
2

IP Address Info →
Look up each subnet to verify its range does not overlap with others. Enter the network address and prefix for each subnet. The tool shows the address range — verify they are contiguous and non-overlapping.
3

Precision Calculator →
Calculate subnet sizes: a /25 has 2^(32-25)-2 = 126 hosts, /26 has 62, /27 has 30, /28 has 14. Sum the sizes to verify they fit in the parent: 128+64+32+32+16+16 = 256 = /24. Plan allocations from largest to smallest to minimize wasted addresses.

Frequently asked questions

Why use VLSM instead of fixed-size subnets?: Fixed-size subnets (FLSM) waste addresses — a /24 for a point-to-point link wastes 252 addresses. VLSM allocates right-sized subnets: /30 for point-to-point, /27 for a small office, /24 for a large department. This can save 50%+ of address space.
How do I align subnets?: A /26 must start at an address divisible by 64 (the block size). 192.168.1.0/26, 192.168.1.64/26, 192.168.1.128/26, 192.168.1.192/26 are valid. 192.168.1.32/26 is invalid — it crosses the 192.168.1.64 boundary. Calculate block size as 2^(32-prefix) and align to multiples of it.