Planning IPv4 Address Space with VLSM
A practical way to allocate differently sized subnets, reduce waste and leave room for growth without painting the network into a corner.
Why fixed-size subnetting wastes space
If every segment receives the same subnet size, the largest requirement determines how much space every segment consumes. That is simple, but often wasteful. A user VLAN needing 200 addresses and a point-to-point segment needing two addresses do not need equal blocks.
VLSM solves that by allowing different prefix lengths inside the same parent allocation. Modern classless routing carries the prefix length with each route, so there is no technical need to keep all subnets the same size.
Allocate largest requirements first
The standard planning method is to sort requested host counts from largest to smallest and allocate each one on the next valid boundary. Large subnets have stricter alignment requirements, so placing them first reduces fragmentation.
Suppose a /22 must support segments needing 120, 60, 30 and 12 hosts. Those fit into /25, /26, /27 and /28 networks respectively. Allocating in that order produces a clean plan and leaves the remainder contiguous for future use.
Plan for more than endpoint count
A host count is only the starting point. Consider gateway addresses, first-hop redundancy, infrastructure appliances, DHCP reservations, monitoring addresses and expected growth. If a segment has 60 devices today, a /26 with 62 usable traditional host addresses leaves almost no operating room.
Also consider the operational cost of renumbering. Saving a few addresses now may not be worth a future maintenance project across hundreds of endpoints.
Keep an address ledger
A VLSM plan should be documented in IPAM or another authoritative source. Track the parent block, each child prefix, purpose, site, VRF, VLAN, gateway and status. Reserve unused space explicitly rather than leaving it as an undocumented gap.
Good records also make route summarization easier. If related networks are kept contiguous, you can often advertise a smaller number of aggregates between sites or routing domains.
Check for collisions before implementation
Before assigning a new subnet, compare it against existing routes, VPN pools, management ranges, cloud VPCs and customer-provided address space. Private addressing does not make a range globally unique.
During mergers and network integrations, duplicate RFC1918 space is common enough that an overlap check should be part of the standard design review.