3Com CoreBuilder 3500, Implementation Manual

Page: 268 / 592

268 C
HAPTER
11: I
NTERNET
P
ROTOCOL
(IP)
Variable Length
Subnet Masks
(VLSMs)
With Variable Length Subnet Masks (VLSMs), each subnet under a
network can use its own subnet mask. Therefore, with VLSM, you can get
more subnet space out of your assigned IP address space.
How VLSMs Work
VLSMs get beyond the restriction that a single subnet mask imposes on
the network. One subnet mask per IP network address fixes the number
of subnets and the number of hosts per subnet.
For example, if you decide to configure the 158.100.0.0/16 network with
a /23 extended-network-prefix, you can create 128 subnets with each
having up to 510 hosts. If some of the subnets do not need that many
hosts, you would assign many host IP addresses but not use them.
With VLSMs, you can assign another subnet mask, for instance, /27, to
the same IP address. So you can assign a longer subnet mask that
consequently uses fewer host IP addresses. As a result, routing tables are
smaller and more efficient.
This method of further subdividing addresses using VLSMs is being used
increasingly more as networks grow in size and number. However, be
aware that this method of addressing can greatly increase your network
maintenance and the risk of creating erroneous addresses unless you plan
the addressing scheme properly.
Route Aggregation
Route aggregation is a numbering scheme in which you can significantly
reduce the total number of IP addresses that your organization uses.
With route aggregation, if you set up the IP address numbering scheme,
including VLSMs, for the network properly, one network advertisement
can reach all of the subnets that report to that network. Similarly, one
subnet broadcast address can reach the subnets that report to the
subnet. In this way, IP traffic that is addressed to a specific part of the
network is advertised only to that part of the network.
For example, the Router ABC in Figure 46 can summarize any number of
subnets that report to it into a single advertisement, and advertise a
single route into the Internet routing table: 78.0.0.0/8. Similarly, the
subnet 78.1.0.0 can advertise to its subnets with one advertisement, and
can summarize advertisements from its subnets by means of a single
advertisement.