When was vlsm introduced

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Gracy Kpopper. Active 1 year, 4 months ago. Viewed 90 times. Improve this question. Giuseppe Giuseppe 25 5 5 bronze badges. Add a comment. Active Oldest Votes. Improve this answer. Community Bot 1. Zac67 Zac67 In a pure classful network, netmasks are not specified -- they are determined by the address. Thus until VLSM came about subnetting was not possible. Cisco routers do not configure VLSM, enable or disable it, or need any configuration to use it. From a configuration perspective, VLSM is simply a side effect of using the ip address interface subcommand.

Routers collectively configure VLSM by virtue of having IP addresses in the same classful network but with different masks. For example, Example shows two of the interfaces from router Yosemite from Figure The use of VLSM can also be detected by a detailed look at the output of the show ip route command. This command lists routes in groups, by classful network, so that you see all the subnets of a single Class A, B, or C network all in a row. Just look down the list, and look to see, if any, how many different masks are listed.

So ends the discussion of VLSM as an end to itself. This chapter is devoted to VLSM, but it took a mere three to four pages to fully describe it. Why the entire VLSM chapter? To do these same tasks on the exam requires skill and practice. 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.

Now that you know the benefit of VLSM, take a look at how you can use it in a network. When designing a network using VLSM, the following simple steps can help come up with an appropriate addressing scheme:. Consider the example shown in Figure and work through the above steps to see how the network address and subnet mask was found for each segment:. Consider Figure as another example.

Using a class C network of Home Guides About Blog Shop. Figure Classful Network Earlier, it was required to use the same subnet mask across the network.