What a VLSM calculator does that a subnet calculator does not
A regular subnet calculator answers a question about one network: given 10.0.0.0/16, what is the mask, the range, the broadcast. Planning a site is the opposite problem. You have one block and a list of needs, a sales floor with 500 devices, a lab with 120, guest Wi-Fi with 60, a router link with two, and you need to carve the block so each gets a network of the right size without overlaps or wasted gaps.
That is variable length subnet masking, or VLSM, and a VLSM calculator does the carving. Subnet Master sizes each network to the smallest prefix that holds its hosts, then allocates them biggest first. Order matters: put a /30 at the start of a block and the /23 that follows can no longer begin on its required boundary, so space is lost. Sorting by size is what lets every network sit flush against the last.
The plan updates as you type. Each row shows the network, its name, the prefix and the spare addresses inside it, so an oversized allocation stands out before it reaches a router.
VLSM subnetting example, worked by the app
Take 192.168.10.0/24 and five needs: Office 60 hosts, Lab 28, Guest 12, Cameras 12, Uplink 2. Choose By host counts, type the block into Block to divide, and use Add a network for each row. The result:
The plan totals 132 addresses, about half of the /24, and every network begins where the previous one ended. Copy all then puts the five rows on the clipboard with netmask, first and last host, broadcast and usable count for each.
- Office: 192.168.10.0/26, 62 usable
- Lab: 192.168.10.64/27, 30 usable
- Guest: 192.168.10.96/28, 14 usable
- Cameras: 192.168.10.112/28, 14 usable
- Uplink: 192.168.10.128/30, 2 usable
- Free space: 192.168.10.132/30, .136/29, .144/28, .160/27 and .192/26
Leftover space as CIDR blocks you can actually assign
In that example 124 addresses remain, but a number like that is no help when the next request arrives. A subnet has to start on a multiple of its own size, so the app breaks what is left into the largest aligned pieces, exactly the five blocks listed above. Hand the /26 to a new department tomorrow and the plan still holds.
If a need will not fit at all, the app names it instead of leaving it off the list, and a utilization bar shows how much of the parent block the plan consumes.
Split a subnet into equal parts
For a quick split, choose Equal parts and enter the Number of subnets. Prefixes divide by halves, so asking for 6 gives 8 and the app says why. Very large splits list the first 512 networks and count the rest.
Under Settings, Reserve a network and broadcast address controls point-to-point links: turned off, a two-host network becomes a /31 with both addresses usable, as RFC 3021 defines. Show the usable range and Show the broadcast address tidy each row to what your firewall or router config needs.
