VLSM calculator: subnets of different sizes
Variable-length subnet masking gives each department a subnet of the size it needs instead of the same size for everyone. Enter the network you have and one line per subnet with a name and a number of hosts, for example “Sales 100”. The largest subnets are placed first, so nothing overlaps and little is wasted.
240 addresses used, 16 still free at the end of the network.
| Name | Hosts needed | CIDR block | Subnet mask | Range | Broadcast | Usable hosts |
|---|---|---|---|---|---|---|
| Sales | 100 | 192.168.0.0/25 | 255.255.255.128 | 192.168.0.1 – 192.168.0.126 | 192.168.0.127 | 126 |
| Office | 50 | 192.168.0.128/26 | 255.255.255.192 | 192.168.0.129 – 192.168.0.190 | 192.168.0.191 | 62 |
| Lab | 20 | 192.168.0.192/27 | 255.255.255.224 | 192.168.0.193 – 192.168.0.222 | 192.168.0.223 | 30 |
| Servers | 10 | 192.168.0.224/28 | 255.255.255.240 | 192.168.0.225 – 192.168.0.238 | 192.168.0.239 | 14 |
Questions
What is VLSM?
Variable-length subnet masking: splitting one network into subnets of different sizes. A team of 100 gets a /25, a team of 20 gets a /27, instead of both getting the same size.
Why are the largest subnets placed first?
A subnet must start at an address that is a multiple of its size. Placing the large ones first keeps the smaller ones neatly after them and avoids holes between subnets.
Why does each subnet have more addresses than I asked for?
Sizes are powers of two, and two addresses per subnet are reserved. For 100 hosts the next size is a /25 with 126 usable hosts. The spare addresses are room to grow.