3Com 4007, Руководство по внедрению, Страница: 364 / 684

364 C
HAPTER
16: IP R
OUTING
The subnet mask 255.255.255.255 is reserved as the default broadcast
address.
Variable Length
Subnet Masks
(VLSMs)
With Variable Length Subnet Masks (VLSMs), each subnetwork under a
network can use its own subnet mask. Therefore, with VLSM, you can get
more subnetwork 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 subnetworks and the number of hosts per subnetwork.
For example, if you decide to configure the 158.100.0.0/16 network with
a /23 extended-network prefix, you can create 128 subnetworks with
each having up to 510 hosts. If some of the subnetworks 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.
Guidelines for Using VLSMs
Consider the following guidelines when you implement VLSMs:
■
When you design the subnetwork scheme for your network, do not
estimate the number of subnetworks and hosts that you need. Work
from the top down until you are sure that you have accounted for all
the hosts, present and future, that you need.
■
Use Open Shortest Path First (OSPF) to carry the extended network
prefix information with each route advertisement.