Multiple Subnets and VLSM Planning

To calculate VLSM subnets, start with a base network and list each subnet's host requirement. Sort the requirements from largest to smallest, then choose the smallest subnet size that fits each one. For example, 60 hosts need a /26 because 2^6 = 64 total addresses and usually 62 usable. Then allocate the next subnet on the next correct boundary. VLSM is really about reducing waste. It lets a 60-host VLAN, a 25-host VLAN, and a 6-host link use different mask sizes instead of forcing one size everywhere.

To split a network into multiple subnets, decide whether the subnets need equal size or different sizes. Equal subnetting is simpler: one prefix length for every subnet. VLSM is more efficient because each subnet is sized by host need. Enter the parent network, such as 10.10.0.0/24, and the required host counts. The output should show subnet address, mask, usable range, broadcast, and available hosts. Always document which VLAN, department, or site receives each subnet, because clean subnet math is only useful when operations can follow it.

Subnetting based on host requirements means you choose the mask after calculating how many addresses each group needs. If a VLAN needs 50 hosts, a /26 is suitable because it provides 62 usable IPv4 host addresses in normal LAN design. If another VLAN needs 12 hosts, a /28 gives 14 usable. This is the heart of VLSM. Instead of wasting a /24 on every department, you allocate just enough space with room for reasonable growth. I usually add a small buffer, not a huge one.

To calculate the subnet mask for each VLSM subnet, find the number of host bits required. The formula is 2^host_bits - 2 usable hosts for normal IPv4 LANs. If you need 30 hosts, 5 host bits gives 30 usable, so the prefix is /27 and the mask is 255.255.255.224. If you need 31 hosts, /27 is no longer enough; use /26. This small jump surprises students. The calculator helps by showing the mask, wildcard, block size, and usable range for each subnet.

To avoid wasting IP addresses, use VLSM and allocate larger subnets first. If you place small subnets randomly, you can fragment the parent network and make it impossible to fit a larger requirement later. For example, assign the 100-host subnet before the 12-host subnet. Also avoid giving every VLAN a /24 just because it is easy. That habit wastes space and creates messy route tables. Good planning leaves sensible growth room, keeps boundaries clean, and makes summarization easier when the site grows.

For VLAN subnet planning with VLSM, list each VLAN name, purpose, gateway, host count, and growth estimate. Then allocate subnets from the parent block, largest host requirement first. The output should become part of your network documentation: VLAN ID, subnet, mask, gateway, DHCP range, exclusions, and summary route if used. For example, users may get a /25, printers a /27, and point-to-point links a /30 or /31 depending on design. Good VLSM planning prevents overlap and makes troubleshooting much easier.

VLSM subnets should usually be calculated from the largest host requirement to the smallest. This keeps the biggest blocks aligned and reduces fragmentation inside the parent network. For example, allocate a 120-host subnet before a 10-host subnet. After each allocation, move to the next correct subnet boundary and continue. This order is a practical habit, not just a classroom rule. It helps preserve clean summaries and avoids the frustrating situation where enough addresses exist overall, but not in one continuous block large enough for the next subnet.