What Is VLSM? (Variable Length Subnet Masking)

WHAT IS VLSM?_

Stop giving a two-host link the same space as a hundred-host LAN. VLSM sizes every subnet to what it actually needs.

[ ▶ SHORT ANSWER ] VLSM (Variable Length Subnet Masking) means using more than one subnet mask length inside the same major network, so each subnet is sized to the number of hosts it actually needs instead of forcing every subnet to be the same size. Instead of splitting a /24 into eight equal /27s, VLSM lets you carve a /25 for a 100-host LAN, a /27 for a 30-host LAN, and a /30 for a point-to-point link — all from the same /24 — wasting almost no addresses. The method: list your subnets largest to smallest, assign the biggest first, then keep subdividing the leftover space. VLSM requires classless routing (a protocol that carries the subnet mask, like OSPF or EIGRP).

Old-school "fixed-length" subnetting splits a network into equal pieces — every subnet gets the same mask. That's wasteful: a link between two routers needs just 2 usable addresses, but a fixed /27 hands it 30. VLSM fixes that by letting subnets have different mask lengths.

The problem VLSM solves

Imagine you have 192.168.1.0/24 and these needs: a 100-host LAN, a 50-host LAN, a 25-host LAN, and two point-to-point router links (2 hosts each). Fixed-length subnetting forces one mask for all of them — and the biggest subnet (100 hosts) needs a /25, so every subnet would have to be a /25. You'd run out after two subnets. VLSM lets each be exactly as big as it needs.

How to do VLSM (largest first)

Always allocate from biggest to smallest so blocks line up and never overlap:

Network: 192.168.1.0/24

100 hosts -> /25  192.168.1.0/25     (.0   - .127)
 50 hosts -> /26  192.168.1.128/26   (.128 - .191)
 25 hosts -> /27  192.168.1.192/27   (.192 - .223)
  2 hosts -> /30  192.168.1.224/30   (.224 - .227)
  2 hosts -> /30  192.168.1.228/30   (.228 - .231)
                  ... .232 - .255 still free

One /24 covered every requirement and left room to grow. That's the whole point.

VLSM vs CIDR

They're related and both "classless," but not the same. VLSM is subnetting within your network using varied masks. CIDR is the bigger concept — classless addressing and route aggregation (summarizing many networks into one prefix) used across the internet. Think of VLSM as CIDR applied inside one organization.

Requirements

Because subnets no longer share a single mask, your routing protocol must carry the mask with each route. That means a classless protocol — OSPF, EIGRP, RIPv2 — and on legacy gear, turning off auto-summary. Classful protocols (RIPv1) can't do VLSM.

★ RELATED QUESTIONS

[ What is the difference between VLSM and CIDR? ] Both are classless. VLSM is subnetting within your own network using different mask lengths. CIDR is the broader idea of classless addressing and route aggregation (summarizing many networks into one prefix) used across the internet. VLSM is essentially CIDR applied inside an organization.

[ Why use VLSM? ] To stop wasting addresses. A point-to-point link needs only two usable hosts (a /30), while a user LAN may need a hundred. VLSM sizes each subnet to fit, instead of giving every subnet the same large mask.

[ How do you calculate VLSM? ] Sort the subnets by host count, largest first. Give the largest the smallest mask that still fits its hosts, then take the next free block and repeat down to the smallest (often a /30 for links). Always allocate biggest to smallest so blocks don't overlap.

WHAT IS VLSM?_

The problem VLSM solves

How to do VLSM (largest first)

VLSM vs CIDR

Requirements

★ RELATED QUESTIONS

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