VLSM

Variable Length Subnet Masks (VLSM)

It gets more complicated than the examples given so far.

Suppose I want to break up a network into subnets of different sizes. I have assigned a class C address to each remote office and I now need to break up each office into separate networks, for security reasons.

Suppose I need the following subnets as shown in table 3.1 below.

Table 3.1

The first step will be to figure out what size subnet will support that many hosts. In table 3.2, we take the information relating to the node size of the required networks and using the network sizes defined in table 3.1, find the most appropriate network size.

Table 3.2

We have also included the subnet masks as we will need this information when configuring nodes and router interfaces. Its easiest to assign the largest subnet first, the last step will be to do this in table 3.3.

Table 3.3

This leaves the address space above .100 free and usable for future use.

If this was a real site, I’d keep the space 128-255 for future use, its half my available space. The subnetting of network 3 as a /30 is a bit messy, it makes subnets .100-.127 awkward to calculate. I might discuss other options for address space above .95 and find a better solution, but that would depend on what the next requirements for this site would be.