Editing Introduction to Classless Routing (section)

==Calculating subnets with VSLM.==
VLSM helps to manage IP addresses. A subnet mask should satisfy the requirements of a LAN with one subnet mask and the requirements of a point-to-point WAN with another. Lets start with an example with a Class B 172.16.0.0 and two LAN that requires at least 250 host. If the router uses a clasfull subnet the WAN link must be a subnet of the same class B Network. Without VSLM the WAN link would need the same subnet mask as the LAN segments. A 24 bits mask of 255.255.255.0 can support 250 host.
This WAN link only needs two addresses, one for each router, meaning that 252 addresses will be wasted.
In this case we can use VSLM and used a subnet mask of /30 bits , since only two addresses are need it.  
<blockquote>
[[Image:LAN3.JPG|Lan needs 250 host each side]]
</blockquote>


Summarization is the number of common bits that the route
will adverstized for the different subnets.