IPv4 Subnet Split and VLSM Planner

Variable Length Subnet Masking (VLSM) and subnet splitting allow network engineers to partition a single parent IPv4 block into multiple child VLAN segments tailored to the actual number of hosts required on each network. Under traditional fixed-length subnetting, every child subnet must share the exact same prefix length. As a result, a point-to-point router interconnect that only needs two IP addresses would consume the same address block size as an office floor with a hundred workstations, rapidly exhausting private or public IPv4 space. This planner supports two complementary allocation modes. Mode A (Equal Split) borrows the required number of host bits to divide a parent block into N equal-sized subnets. Mode B (VLSM Planner) takes a list of variable host requirements, sorts them strictly from largest block size to smallest, and packs each child subnet along its natural power-of-two boundary. Sorting from largest to smallest is mandatory in CIDR arithmetic because every prefix block must start on an integer multiple of its block size; placing a small /30 block at the start of a /24 network would prevent an immediately adjacent /25 block from aligning without leaving a fragmented gap. Common mistakes during manual VLSM planning include allocating subnets in VLAN ID order instead of descending host count order, forgetting to subtract the two reserved addresses (Network and Broadcast) when sizing blocks such as 30 hosts into a /27 (which only supports 30 usable hosts with zero growth margin), and failing to document remaining unallocated space. Exporting the resulting table directly to CSV or Markdown keeps IP address management (IPAM) documentation clean and verifiable.