What is vlsm and how is it used

The rest of this chapter examines the skills to apply VLSM and provides some practice for these two key areas:. Regardless of whether a design uses VLSM, the subnets used in any IP internetwork design should not overlap their address ranges. As a result, hosts in different locations can be assigned the same IP address.

Routers clearly cannot route packets correctly in these cases. In short, a design that uses overlapping subnets is considered to be an incorrect design and should not be used. It then gets into an operational and troubleshooting approach to the topic, by looking at existing designs and trying to find any existing overlaps.

When creating a subnetting plan using VLSM, you have to be much more careful in choosing what subnets to use. For example, consider a subnet plan for Class B network If you use the math and processes to find all subnet IDs per Chapter 21, all those subnet IDs happen to have binary 0s in the host fields. To begin, you would decide that you need some subnets with one mask, other subnets with another mask, and so on, to meet the requirements for different sizes of different subnets.

You might develop then a planning diagram, or at least draw the ideas, with something like Figure The drawing shows the first few subnet IDs available with each mask, but you cannot use all subnets from all three lists in a design. As soon as you choose to use one subnet from any column, you remove some subnets from the other lists because subnets cannot overlap. Overlapping subnets are subnets whose range of addresses include some of the same addresses.

However, it shows a check mark beside two subnets that have been allocated for use; that is, on paper, the person making the subnetting plan has decided to use these two subnets somewhere in the network.

The subnets with a dark gray shading and an X in them can no longer be used because they have some overlapping addresses with the subnets that have check marks Just to complete the example, first look at subnet That subnet includes addresses from the subnet ID of As you can see just by looking at the subnet IDs to the right, all the subnets referenced with the arrowed lines are within that same range of addresses.

Now look to the upper right of the figure, to subnet The subnet has a range of That subnet overlaps with the two subnets referenced to the left. For instance, subnet But because there is some overlap, once the design has allocated the A subnetting design, whether using VLSM or not, should not allow subnets whose address ranges overlap. If overlapping subnets are implemented, routing problems occur and some hosts simply cannot communicate outside their subnets.

These address overlaps are easier to see when not using VLSM. With VLSM, overlapped subnets may not have the same subnet ID, as was the case in this most recent example with the subnets across the top of Figure To find these overlaps, you have to look at the entire range of addresses in each subnet, from subnet ID to subnet broadcast address, and compare the range to the other subnets in the design.

It uses a single Class B network Now imagine that the exam question shows you the figure, and either directly or indirectly asks whether overlapping subnets exist. This type of question might simply tell you that some hosts cannot ping each other, or it might not even mention that the root cause could be that some of the subnets overlap.

To answer such a question, you could follow this simple but possibly laborious process:. Step 1. Calculate the subnet ID and subnet broadcast address of each subnet, which gives you the range of addresses in that subnet. Step 2. List the subnet IDs in numerical order along with their subnet broadcast addresses.

Step 3. Scan the list from top to bottom, comparing each pair of adjacent entries, to see whether their range of addresses overlaps. For example, Table completes the first two steps based on Figure , listing the subnet IDs and subnet broadcast addresses, in numerical order based on the subnet IDs. As for the process, Step 3 states the somewhat obvious step of comparing the address ranges to see whether any overlaps occur. Note that, in this case, none of the subnet numbers are identical, but two entries highlighted do overlap.

The design is invalid because of the overlap, and one of these two subnets would need to be changed. As far as the three-step process works, note that if two adjacent entries in the list overlap, compare three entries at the next step.

The two subnets already marked as overlapped can overlap with the next subnet in the list. For example, the three subnets in the following list overlap in that the first subnet overlaps with the second and third subnets in the list. If you followed the process shown here, you would have first noticed the overlap between the first two subnets in the list, so you would then also need to check the next subnet in the list to find out if it overlapped.

As typical of anything to with applying IP addressing and subnetting, practice helps. To that end, Table lists three practice problems. Just start with the five IP addresses listed in a single column, and then follow the three-step process outlined in the previous section to find any VLSM overlaps. The task described in this section happens frequently in real networks: choosing new subnets to add to an existing design.

In other words, you need to pick a new subnet and not make a mistake! For example, consider the internetwork shown earlier in Figure , with classful network So, you really have a couple of tasks: To find all the subnet IDs that could be used, rule out the ones that would cause an overlap, and then check to see whether the question guides you to pick either the numerically lowest or highest subnet ID.

This list outlines the specific steps:. Pick the subnet mask prefix length for the new subnet, based on the design requirements if not already listed as part of the question. Calculate all possible subnet numbers of the classful network using the mask from Step 1, along with the subnet broadcast addresses. Make a list of existing subnet IDs and matching subnet broadcast addresses.

Step 4. Compare the existing subnets to the candidate new subnets to rule out overlapping new subnets. Step 5. Choose the new subnet ID from the remaining subnets identified at Step 4, paying attention to whether the question asks for the numerically lowest or numerically highest subnet ID. In this case, you need to add a new subnet to support hosts. Imagine that the question tells you to use the smallest subnet least number of hosts to meet that requirement.

At this point, just follow the steps listed before Figure For Step 2, you need to list all the subnet numbers and broadcast addresses of You will not use all these subnets, but you need the list for comparison to the existing subnets.

Next, at Step 3, list the existing subnet numbers and broadcast addresses, as shown earlier in Figure Save Article. Improve Article.

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Lets begin with HQ with 50 hosts, using the table above:. We are borrowing 2 bits with value of This is the closest we can get for 50 hosts. HQ — Total address space HQ Network Mask HQ address will look like this We are borrowing 3 bits with value of 32; this again is the closest we can get to the number of host needed.

RO1 address will start from