VLSM planning
Allocate variable-length subnets inside a parent block without overlap, verify capacity and alignment, and keep space for growth.
VLSM method
VLSM assigns different prefix lengths inside one parent block so each subnet gets just the space it needs. The method is always the same: sort requirements largest-first, allocate aligned blocks, and verify that no two allocations overlap and the largest request still fits.
VLSM allocation procedure
- Sort the subnet requirements by host count, largest first.
- For each need, pick the smallest prefix whose usable capacity covers the hosts.
- Allocate the next free aligned block — the network address must be a multiple of the block size.
- Track used space and remaining free space; leave headroom for growth.
- Verify every allocation: aligned, contained in the parent, non-overlapping, sufficient.
Host needs to smallest prefix
| Hosts needed | Smallest prefix | Usable capacity | Addresses used |
|---|---|---|---|
| 1 | /32 or /31 p2p | 1 / 2 | 1 / 2 |
| 2 | /30 | 2 | 4 |
| 3–6 | /29 | 6 | 8 |
| 7–14 | /28 | 14 | 16 |
| 15–30 | /27 | 30 | 32 |
| 31–62 | /26 | 62 | 64 |
| 63–126 | /25 | 126 | 128 |
| 127–254 | /24 | 254 | 256 |
Allocating largest-first keeps blocks contiguous and leaves room to grow.
Worked allocation
- Needs
- Sales 90 hosts, Eng 50, Ops 20, WAN link 2
- Parent
- 192.168.10.0/24
- Sort largest first: 90 → needs /25 (126 usable); 50 → /26 (62); 20 → /27 (30); 2 → /30 or /31.
- Allocate /25 first: 192.168.10.0/25 → .0–.127 used, .128 free.
- Allocate /26 next: 192.168.10.128/26 → .128–.191, next free .192.
- Allocate /27: 192.168.10.192/27 → .192–.223, next free .224.
- Allocate the WAN /30: 192.168.10.224/30 → .224–.227 (or /31 for a true point-to-point).
Result: .0/25 Sales, .128/26 Eng, .192/27 Ops, .224/30 WAN — non-overlapping, aligned, and .228–.255 still free.
Verification checklist
- Alignment: the network address must be a multiple of the block size — 192.168.10.64/26 is valid, 192.168.10.96/26 is not (96 is not a multiple of 64).
- Containment: every allocation must fit inside the parent block.
- Non-overlap: no address may appear in two allocations.
- Capacity: usable hosts ≥ required hosts, or the allocation fails at deploy time.
Mistakes and verification
Common mistakes
- Allocating small subnets first — fragmentation strands large needs between small blocks.
- Starting an allocation at an unaligned address (e.g. .96/26).
- Sizing by raw addresses instead of usable hosts (a /28 has 16 addresses but only 14 hosts).
- Ignoring growth — filling a parent block to 100% leaves no room to split later.
Observation: Two new subnets reject with overlapping-address errors on the router.
- Second allocation started before the first block ended
- Prefix length changed without recomputing the boundary
- Parent block misidentified
| Check | Indicates |
|---|---|
| List each allocation with its exact first/last address | Overlap is visible immediately in a written table. |
| Recompute each block boundary from its mask | A /26 starting at .96 is unaligned — the real block is .64–.127. |
Correction: Reallocate starting at the correct aligned boundary.
Router accepts both subnets; show ip interface brief lists both with no overlap warning.
Related resources
Sources
- Cisco CCNA 200-301 v1.1 Exam Topics (official PDF) — reviewed 2026-11-02
- RFC 4632 — Classless Inter-domain Routing (CIDR) — reviewed 2026-11-02
Targets CCNA 200-301 v1.1 objectives. Supplemental blocks are labeled beyond exam scope.