Classless Routing

Classless Routing
Classless routing, also known as classless interdomain routing (CIDR), is not dependent on the
default boundaries of the different classes of IP addresses. Classless routing actually allows each
route’s subnet mask to be sent in the routing update with the route. Classless routing also opens
the door for variable-length subnet masking (VLSM), which extends IP addressing beyond the
limitations of using fixed-length subnet masks (FLSMs) by allowing you to specify the most efficient
subnet mask to use for the size of network in question. This allows you to conserve IP
addresses, extending the use of IP address space. This topic is covered in more detail in Chapter 3.
To simplify the difference between classless and classful routing, let’s use an example of a college
campus. The college campus is built with buildings of identical size. It doesn’t matter how many
offices in each building will be occupied; all buildings are the same size and every office has to have
a number. This is analogous to a classful network design, where every host has a host ID and participates
in the same size network, regardless of how many hosts will ever really be on that network.
The addresses that are not used on a network cannot be used on a different network that is running
short of addresses. All networks will have to grow the same amount to be able to cover the largest
need. All buildings have to remain identical in size, even if those that are not full must grow to keep
up with the growth of the fullest building. All of the wasted office space is just that. Each group in
a building is confined to that building and cannot grow into the less-populated buildings.
Introducing classless routing would be like allowing each building to be only as large as the
group within the building, wasting no office space and having only enough empty offices to
cover projected growth. In other words, classless routing leads to the ability to use not only subnet
masks that are not the default masks, but also to use VLSM—subnet masks of different
sizes—so that address waste is minimized.
The routing protocols we cover in this book that are considered classless routing protocols
are the following:
 RIPv2
 EIGRP
 OSPF
Components of Routing Data 19
 IS-IS
 BGP
So far, we have described how routes are learned, the different classes of routing protocols, and
the different categories of routing protocols. Now that you have a firm grasp on these concepts,
it’s time to move on to how these routes you’ve learned actually get placed in a routing table.