TCP/IP Configuration and Management Manual

NonStop TCP/IP Processes and Protocols
TCP/IP Configuration and Management Manual—427132-004
B-7
The Problem of Resolving Addresses
Subnet Addressing Advantages
There are many advantages of using subnet addressing. For one thing, it is
transparent to the rest of the Internet. That is, the rest of the Internet does not know or
care whether you have implemented subnet addressing on your own network. All
routing and delivery mechanisms remain unchanged on the Internet.
Another advantage is that subnets may also be assigned and administered by the local
administration authority. When you are provided with a network address, you may
assign the subnet structure to suit your particular needs. As administrator of your local
network, you design the subnet structure to support the networks and hosts on your
network. This is discussed in more detail.
Another advantage of subnetting is that the existence of subnets does not increase or
decrease the number of hosts that may be connected to any particular network.
Consequently, if you are assigned a Class C network address, you are able to connect
a total of 254 hosts to that network regardless of whether or not you have implemented
subnet addressing.
In order to make this concept of subnet addressing more tangible, let’s look at an
example of subnetting. As you know, there are three primary classes of Internet (IP)
addresses: A, B, and C. As we have seen, the difference between these classes is that
the network address and the local address are different sizes. Class A addresses allow
one octet for the network address and three octets for the local address; Class B
addresses allow two octets for the network address and two octets for the local
address; and Class C addresses all three octets for the network address and one octet
for the local address.
Subnetting can be used with all of the network address classes even though only Class
B addresses are used in the examples that follow. Figure 6-1 illustrates the use of a
Class B address. Notice how subnetting allows you to split up the two octets of the
local address into an actual physical network address (128) and a local address (3).
Using the previous example, the physical network could be arranged like the network
shown inFigure B-2
.
This illustration shows how the two octets of the local address can be split so that the
first octet specifies the physical network address (128, 64, and 192 respectively) while
the second octet is used to specify the physical host address.
These different physical networks could be in the next room, an adjoining building or
across the country. It therefore makes sense to subnet a network when there are
geographic or administrative reasons to do so.
You can see from this example that from a single Class B network address, the
administrator can separate the last two octets of the local address into a physical
network address and a local host address. The same concept applies to any class of
Internet addresses you may have.