VLSM(Variable Length Subnet Masks)

VLSM Step by Step

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:

• more efficient use of addressing
• Ability to perform route summarization

when you perform classful subnetting, 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.

• Find the largest segment in the area—the segment with the largest number of devices connected to it.
• Find the appropriate subnet mask for the largest network segment.
• Write down your subnet numbers to fit your subnet mask.
• For your smaller segments, take one of these newly created subnets and apply a different, more appropriate, subnet mask to it.
• Write down your newly subnetted subnets.

For even smaller segments, go back to step 4.

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. The bigger 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:

• It reduces the size of routing tables, requiring less memory and processing.
• It reduces the size of updates, requiring less bandwidth.
• It contains network problems

EXAMPLE OF VLSM

Above image shows several branch offices using subnetted Class C (/26) addresses that provide each branch with 62 possible host IPs. The branches are connected to the central office via point-to-point WAN links. The ideal mask to use for such a link is /30 because it provides only 2 hosts, one for each end of the link. The problem arises when the routing protocols are configured: Prior to VLSM, the /30 networks could not be used because the /26 networks existed in the same system and the classful routing protocols could only advertise one mask per class of address. All networks, including the little /30 links, had to use the same mask of /26. This wastes 60 IP addresses on each WAN link.

With the implementation of VLSM-capable routing protocols, we can deploy a /30 mask on the point-to-point links, and the routing protocols can advertise them as /30s along with the /26s in the branches because the subnet mask for each network is included in the routing updates.

VLSM has allowed us to make the point-to-point link networks the ideal size (two hosts on each) using /30 masks. This has allowed us to use a single subnetted Class C network for all the addressing requirements in this scenario—and as you'll see, it makes a perfect opportunity to summarize these routes. This is what is meant by "more efficient addressing"— in other words, making networks the right size without depleting the limited address space or limiting future growth.

lassless Interdomain Routing (CIDR), specified in RFC 2050, is an extension to VLSM and route summarization.

With VLSM, you can summarize subnets back to the Class A, B, or C network boundary. For example, if you have a Class C network 192.168.1.0/24 and subnet it with a 26-bit mask, you have created four subnets. Using VLSM and summarization, you can summarize these four subnets back to 192.168.1.0/24.

CIDR takes this one step further and allows you to summarize a block of contiguous class A, B, and C network numbers. This practice is commonly referred to as supernetting. Today’s classless protocols support supernetting. However, it is most commonly configured by ISPs on the Internet using BGP.

Discontiguous subnets are not supported by classful protocols but are supported by classless protocols. Classful protocols do not include the subnet mask when advertising network and subnet numbers. When implementing route summarization, another thing you’ll need to consider is that routing decisions, by a router, must be made on the entire destination IP address in the IP packet header. The router always uses the longest matching prefix in the routing table.

CIDR allows you to summarize class networks together; VLSM allows you to summarize subnets only back to the class network boundaryEach segment has a single network number and mask. VLSM allows a class address, not a network segment, to have more than one subnet mask.