As IP subnets have grown, administrators have looked for ways to use their address space more efficiently. One technique is called Variable-Length Subnet Masks (VLSM). With VLSM, a network administrator can use a long mask on networks with few hosts, and a short mask on subnets with many hosts.

In order to use VLSM, a network administrator must use a routing protocol that supports it. Cisco routers support VLSM with Open Shortest Path First (OSPF), Integrated Intermediate System to Intermediate System (Integrated IS-IS), Enhanced Interior Gateway Routing Protocol (EIGRP), RIP v2, and static routing.

VLSM allows an organization to use more than one subnet mask within the same network address space. Implementing VLSM is often referred to as "subnetting a subnet", and can be used to maximize addressing efficiency.

Classful routing protocols require that a single network use the same subnet mask. Therefore, network 192.168.187.0 must use just one subnet mask such as 255.255.255.0.

VLSM is simply a feature that allows a single autonomous system to have networks with different subnet masks. If a routing protocol allows VLSM, use a 30-bit subnet mask on network connections, 255.255.255.252, a 24-bit mask for user networks, 255.255.255.0, or even a 22-bit mask, 255.255.252.0, for networks with up to 1000 users.

In the past, it has been recommended that the first and last subnet not be used. Use of the first subnet, known as subnet zero, for host addressing was discouraged because of the confusion that can occur when a network and a subnet have the same addresses. The same was true with the use of the last subnet, known as the all-ones subnet. It has always been true that these subnets could be used. However, it was not a recommended practice. As networking technologies have evolved, and IP address depletion has become of real concern, it has become acceptable practice to use the first and last subnets in a subnetted network in conjunction with VLSM.
In this network, the network management team has decided to borrow three bits from the host portion of the Class C address that has been selected for this addressing scheme.

If management decides to use subnet zero, it has eight useable subnets. Each may support 30 hosts. If the management decides to use the no ip subnet-zero command, it has seven usable subnets with 30 hosts in each subnet. From Cisco IOS version 12.0, remember that Cisco routers use subnet zero by default. Therefore Sydney, Brisbane, Perth, and Melbourne remote offices may each have 30 hosts. The team realizes that it has to address the three point-to-point WAN links between Sydney, Brisbane, Perth, and Melbourne. If the team uses the three remaining subnets for the WAN links, it will have used all of the available addresses and have no room for growth. The team will also have wasted the 28 host addresses from each subnet to simply address three point-to-point networks. Using this addressing scheme one third of the potential address space will have been wasted.

Such an addressing scheme is fine for a small LAN. However, this addressing scheme is extremely wasteful if using point-to-point connections.

Route aggregation with VLSM

When using VLSM, try to keep the subnetwork numbers grouped together in the network to allow for aggregation. This means keeping networks like 172.16.14.0 and 172.16.15.0 near one another so that the routers need only carry a route for 172.16.14.0/23.

The use of Classless InterDomain Routing (CIDR) and VLSM not only prevents address waste, but also promotes route aggregation, or summarization. Without route summarization, Internet backbone routing would likely have collapsed sometime before 1997.

Route summarization, or supernetting, is only possible if the routers of a network run a classless routing protocol, such as OSPF or EIGRP. Classless routing protocols carry a prefix that consists of 32-bit IP address and bit mask in the routing updates. In Figure , the summary route that eventually reaches the provider contains a 20-bit prefix common to all of the addresses in the organization, 200.199.48.0/22 or 11001000.11000111.0011. For summarization to work properly, carefully assign addresses in a hierarchical fashion so that summarized addresses will share the same high-order bits.

Remember the following rules:

1)A router must know in detail the subnet numbers attached to it.
2)A router does not need to tell other routers about each individual subnet if the router can send one aggregate route for a set of routers.
3) A router using aggregate routes would have fewer entries in its routing table.
VLSM allows for the summarization of routes and increases flexibly by basing the summarization entirely on the higher-order bits shared on the left, even if the networks are not contiguous.

Ref; www.cisco.com/warp/public/779/edu/demos/CCNA3v3Demo/index.html

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