Understanding Variable Length Subnet Masks (VLSMs) Part 2

By Dan DiNicolo, June 3rd, 2006 Posted in Subnetting. Subscribe to our RSS Feed

Let’s start off by breaking our 192.168.0.0/16 network into 6 large subnets, since using the required 4 would only account for current regions, and would not allow for future growth. Doing this is not in any way different that what we’ve done in the past. To get 6 large subnets, we’ll need to steal 3 bits, since 2³-2 gives us 6 subnets. In this case, our new mask becomes 255.255.224.0, or /19. Each of the new subnet IDs is listed in this table, including the location that it will be used for.

Since we’ve used a /19 subnet mask, each of these ranges supports up to 2¹³-2 or 8190 hosts as things presently stand. However, we’re not done. Our next goal is to take one of the areas and further divide it into a number of smaller networks. For example, let’s say that North America includes 16 offices. Of these, 3 are large, and need to support up to 400 hosts. 13 are smaller, and need to support 150 hosts maximum. In the future, it is possible that more large and small offices will be added.

In order to deal with these requirements, we need to subnet the North America network further. The easiest way to deal with this is to consider 192.168.0.0/19 to be just another network ID, and our starting point. To begin, we know that we need to immediately support 3 large offices, each with up to 400 hosts. For this example, I’m going to have us define one additional large office, in order to account for future growth. That gives us a requirement of 4 networks, and 400 hosts per network. Using the host portion, we would need 9 host bits in order to support our requirement, which would give us 2⁹-2 or 510 hosts per subnet. In doing so, we are left with 4 bits to define subnets, as shown. Remember that our starting subnet mask is 255.255.224.0 or /19 in this case.

Using the first 4 available ranges to define our four large LANs, we would allocate the subnet IDs as shown in this table.

Written by Dan DiNicolo - Visit Website

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