To make things interesting, let’s go through what I consider to be a tough subnetting question. To be honest, if you can follow this question and understand what you’re doing, there isn’t a subnetting question I can ask that you can’t answer. We’re going to go through it step-by-step, just to be sure that you’re clear on determining the correct subnet mask, and the ranges of hosts on the first, second, and last subnets.
In this scenario, we’re going to assume a network ID of 10.0.0.0. You should already be familiar with its default mask. Our requirement is to have at least 4000 subnets. Our goal is to determine both the custom subnet mask, as well as the first, second, and last ranges of host IDs.
Let’s start with the custom subnet mask. Since we need 4000 subnets minimum, we need to determine how many bits are required. It should already be obvious that we’ll need to move beyond a single octet, since 8 bits will only provide a maximum of 256 subnets. If we try 10 bits, we come up with a value of 210-2 or 1022 subnets, obviously not enough. 11 bits provides 2046 subnets and 12 bits 4094 (212-2). Obviously 12 bits meets out requirement, so that’s what we’ll use.
Recall that the default subnet mask for the private 10.0.0.0 network address is 255.0.0.0. If we add an additional 12 bits, the mask becomes 255.255.240.0, as shown in the figure below.
Now that we know our custom mask, let’s determine how many hosts we can support per subnet. From the diagram above, we have 12 host bits remaining, giving us a maximum of 212-2, or 4094 hosts per subnet.
Our next goal is determining the first valid range of IP addresses. Don’t get thrown off by the fact that the subnet portion of the address spans one and a half octets. If we simply follow the same rules as before, we’ll be fine. In this case, set all 12 subnet bits the lowest non-zero value, and determine the first and last valid host addresses, as shown in the figure below.
Notice that the first valid range of IP addresses on subnet 10.0.16.0 goes from 10.0.16.1 up to 10.0.31.254. The second subnet will start when we set the 12 bits of the subnet ID to 00000000 0010.
The second valid range of addresses becomes 10.0.32.1 to 10.0.47.254. The broadcast ID for this subnet is 10.0.47.255.
The subnet IDs will continue to increase until the last range is reached – remember that this occurs when the subnet ID is set to the highest value that is not all binary 1 values – in this case, 11111111 1110.
As such, our last valid subnet is subnet ID 10.255.224.0, with an address range from 10.255.224.1 up to 10.255.239.254. Remember that in between the three subnets that we’ve looked at, another 4091 subnets exist.