VLSM Calculator
Give a parent IPv4 network and the host count for each LAN. The planner sizes a subnet for every count, largest first, and stops if the next block would fall outside the parent.
How a block is chosen
Each host count becomes the longest prefix whose usable hosts can hold it. 100 hosts need a /25 (126 usable). 50 need a /26. 20 need a /27. 10 need a /28.
Inside 192.168.0.0/24 those four blocks land at 192.168.0.0/25, 192.168.0.128/26, 192.168.0.192/27, and 192.168.0.224/28. Sixteen addresses, 192.168.0.240 through 192.168.0.255, stay free.
The planner sizes each subnet to the count you type. It does not keep a spare inside that subnet. Fifty hosts fit in a /26, which has 62 usable addresses. If the LAN will grow, type the future count. Space left at the end of the parent is reported. Space you hoped to reserve inside a subnet is not.
Questions
A large block can only start on its own size boundary. Place a small block there first and the large one has to jump ahead, leaving a hole that no later requirement can use.
For /0 through /30 the usable count is the block size minus the network and the broadcast. Two usable hosts need a block of four, which is a /30. A point-to-point link can use a /31 under RFC 3021. This planner does not assign /31s.
No. The planner sorts largest first. Typing 10, 100, 20, 50 produces the same blocks as typing 100, 50, 20, 10. Allocating by hand in the order you first wrote them can strand space. This page does not do that.
They get the same prefix and sit one after the other. Two requests for 50 hosts, in a parent that can hold them, become two /26s.
Sources
Reviewed 8 October 2026. The methodology page records what these labels do not cover.
