VLSM Planner
How it works
What is VLSM? Variable Length Subnet Masking allows dividing a network into subnets of different sizes. Unlike classful or fixed-length subnetting, VLSM right-sizes each subnet, conserving address space.
Why largest-first? Allocating the largest subnets first ensures correct alignment. A /25 block must start on a 128-boundary, a /26 on a 64-boundary, etc. Starting large guarantees these alignment constraints are naturally satisfied.
Understanding the results:
- Allocated = total addresses in the block (always a power of 2)
- Usable = allocated − 2 (network + broadcast)
- Waste = usable − requested (hosts you asked for vs. what the block provides)
Practical example: Subnetting a /24 for an office with Engineering (50 hosts), Marketing (30), Servers (10), Guest Wi-Fi (20), Management VLAN (5), and P2P links (2 each) produces subnets ranging from /25 to /30 — all non-overlapping within the parent.
VLSM and routing protocols: VLSM requires classless routing protocols (OSPF, EIGRP, BGP, RIPv2). RIPv1 does not support VLSM.
Exam tips: VLSM allocation is a common CCNA exam topic. Practice: given a /24 and a list of host requirements, allocate subnets on paper, then verify with this tool.
FAQ
What is VLSM?
Variable Length Subnet Masking (VLSM) allows dividing an IP network into subnets of different sizes. Unlike classful or fixed-length subnetting, VLSM right-sizes each subnet, conserving address space.
How does VLSM allocation work?
VLSM allocates subnets from largest to smallest within a parent network. For each request, it finds the smallest power-of-2 block that fits the required hosts (plus network and broadcast addresses), aligns it on the correct boundary, and assigns it sequentially. This greedy approach minimizes wasted addresses.
Why are VLSM subnets always powers of 2?
IP subnets must be contiguous blocks of addresses aligned to power-of-2 boundaries. This is a fundamental constraint of CIDR routing — a subnet mask can only represent powers of 2. So a request for 100 hosts gets a /25 (128 addresses, 126 usable), not exactly 102.
What happens to unused address space in VLSM?
Addresses not allocated to any subnet remain in the parent network and can be used for future subnets. The planner shows remaining addresses and utilization percentage. Waste within subnets (due to power-of-2 rounding) is also tracked per subnet.