VLSM Calculator — Variable Length Subnet Masking for Efficient IP Allocation

Our free VLSM calculator (Variable Length Subnet Masking) allocates differently sized subnets from a single network block, sized to exactly meet each network's host requirements — minimising wasted IP addresses. Enter your base network in CIDR notation and list your required host counts (largest first) to get an optimally allocated set of subnets. Essential for CCNA exam preparation and real-world network design.

What is VLSM and Why Use It?

Traditional subnetting divides a network into equal-sized pieces. VLSM allows you to divide a network into pieces of different sizes — giving a large office 120 addresses while giving a point-to-point WAN link only 2, all from the same address block. Without VLSM, a company might need ten separate /24 networks when a single carefully designed /21 with VLSM would be more efficient and easier to summarise in routing tables.

VLSM Design Process

• Step 1: List all networks and their host requirements, sorted largest to smallest
• Step 2: For each network, find the smallest subnet that accommodates the required hosts (2ⁿ - 2 ≥ required hosts)
• Step 3: Allocate the subnet starting at the next available address in the block
• Step 4: The next subnet starts immediately after the broadcast address of the previous one
• Step 5: Verify no subnets overlap and sufficient addresses remain in the parent block

VLSM in Modern Networks

VLSM requires a classless routing protocol — OSPF, EIGRP, IS-IS, or BGP — that carries the subnet mask with each route advertisement. Older classful protocols like RIPv1 do not support VLSM. VLSM is also fundamental to AWS VPC design, where you might allocate /20 subnets for large production tiers, /24 for smaller services, and /28 for transit gateway attachments — all from a single /16 VPC CIDR.