VLAN Subnet Planning: Sizing Subnets for Every VLAN
Each VLAN usually gets its own subnet. Planning them together makes sure every VLAN has room to grow and fits cleanly inside a larger block.
One VLAN, one subnet
The common design maps each VLAN to a single subnet, such as Building 12 / VLAN 85 on 10.150.12.0/24. This keeps routing simple and makes troubleshooting predictable.
Size for growth and gateways
Count today's devices, add room for growth, then reserve addresses for the routers. A redundant pair typically uses two physical gateway addresses plus one virtual FHRP address (HSRP, VRRP, or GLBP).
Group by site or building
Give each building or site its own larger block and carve VLAN subnets out of it. Summaries stay clean, and adding a VLAN later doesn't scatter addresses across the network.