VLSM(Variable Length Subnet Mask )

 Variable Length Subnet Mask :

                                                     Neither RIPv1 nor IGRP routing protocols have a field for subnet information, so the subnet information gets dropped. What this means is that if a router running RIP has a subnet mask of a certain value, it assumes that all interfaces within the classful address space have the same subnet mask. This is called classful routing, and RIP and IGRP are both considered classful routing protocols. Classless routing protocols, however, do support the advertisement of subnet information. Therefore, you can use VLSM with routing protocols such as RIPv2, EIGRP, and OSPF. The benefit of this type of network is that you save a bunch of IP address space with it.

 VLSM  enables you to have more than one mask for a given class of address, albeit a class A, B, or C network number.

VLSM, originally defined in RFC 1812, allows you to apply different subnet masks to the same class address space Classful protocols, such as RIPv1 and IGRP, do not support VLSM. To deploy VLSM requires a routing protocol that is classless—BGP, EIGRP, IS-IS, OSPF, or RIPv2, for instance. VLSM provides Two major advantages.

1     .more efficient use of addressing .

2.      Ability to perform route summarization when you perform classfulsubnetting, all subnets have the same number of hosts because they all use the same subnet mask. This leads to inefficiencies. For example, if you borrow 4 bits on a Class C network, you end up with 14 valid subnets of 14 valid hosts. A serial link to another router only needs 2 hosts, but with classical subnetting, you end up wasting 12 of those hosts. Even with the ability to use NAT and private addresses, where you should never run out of addresses in a network design, you still want to ensure that the IP plan that you create is as efficient as possible. An efficient addressing scheme using VLSM.

3. Find the largest segment in the area—the segment with the largest number of devices connected to it.

4. Find the appropriate subnet mask for the largest network segment.

5.  Write down your subnet numbers to fit your subnet mask.

6. For your smaller segments, take one of these newly created subnets and apply a different, more appropriate, subnet mask to it.

7. Write down your newly subnetted subnets.

8. For even smaller segments, go back to step 4.

Route Summarization :

Route summarization is the ability to take a bunch of contiguous network numbers in your routing table and advertise these contiguous routes as a single summarized route. Route summarization, or supernetting, is needed to reduce the number of routes that a router advertises to its neighbor. Remember that for every route you advertise, the size of your update grows. It has been said that if there were no route summarization, the Internet backbone would have warped from the total size of its own routing tables back in 1997. Routing updates, whether done with a distance vector or link-state protocol, grow with the number of routes you need to advertise. In simple terms, a router that needs to advertise ten routes needs ten specific lines in its update packet. The more routes you have to advertise, the bigger the packet. Thebigger the packet, the more bandwidth the update takes, reducing the bandwidth available to transfer data. But with route summarization, you can advertise many routes with only one line in an update packet. This reduces the size of the update, allowing you more bandwidth for data transfer. Summarization allows you to create a more efficient routing environment by providing the following advantages:

1. It reduces the size of routing tables, requiring less memory and processing.

2. It reduces the size of updates, requiring less bandwidth.

3. It contains network problems 

 Example of VLSM  :