Extending IP Addresses
Design Considerations with VLSM
Now that we have seen how valuable VLSMs can be in preserving those
precious IP addresses, be aware that there is a catch. Specifically, if you use
a classful routing protocol (a protocol that advertises routes at the Class A,
Class B, and Class C boundaries) such as RIPv1 or IGRP, then VLSMs are
not going to work.
RIPv1 and IGRP routing protocols do not have a field for subnet infor-
mation. Therefore, the subnet information gets dropped. This means 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.
Classless routing protocols, however, do support the advertisement of sub-
net information. So, you can use VLSM with routing protocols such as
RIPv2, EIGRP, or OSPF.
Another important point to consider when assigning addresses is to not
have discontiguous networks. Specifically, if you have two subnets of the
same network separated by a different network, some of your hosts could
become unreachable. Consider the network shown in Figure 3.4.
F I G U R E 3 . 4
Discontiguous networking example
If routes are being summarized--a technique called route summariza-
tion--then both RouterA and RouterB will be advertising to the WAN cloud
that they are the route to network 172.16.0.0/16. While there are techniques
to overcome this behavior, which will be discussed in the "Route Summari-
zation" section later in this chapter, it makes for better network design to not
separate a network's subnets by another network.
Copyright ©2001 SYBEX , Inc., Alameda, CA