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Intranets
Chapter 10
Subnetting an Intranet
CONTENTS
Intranets come in different sizes. In a small company, an intranet
can be composed of only a handful of computers. In a medium-sized
business, it may include dozens or hundreds of computers. And
in a large corporation, there may be thousands of computers spread
across the globe, all connected to a single intranet. When intranets
get large, they need to be subdivided into individual subnets
or subnetworks.
To understand how subnetting works, you first need to understand
IP addresses. Every IP address is a 32-bit numeric address that
uniquely identifies a network and then a specific host on that
network. The IP address is divided into two sections: the network
section, called the netid, and the host section, called
the hostid.
Each 32-bit IP address is handled differently, according to what
class of network the address refers to. There are three
main classes of network addresses: Class A, Class B,
and Class C. In some classes, more of the 32-bit address
space is devoted to the netid, while in others, more of the address
space is devoted to the hostid. In a Class A network, the netid
is composed of 8 bits, while the hostid is composed of 24 bits.
In a Class B network, both the netid and the hostid are composed
of 16 bits. In a Class C network, the netid is composed of 24
bits, while the hostid is composed of 8 bits. There's a simple
way of knowing what class a network is in. If the first number
of the IP address is less than 128, the network is a Class A address.
If the first number is from 128 to 191, it's a Class B network.
If the first number is from 192 to 223, it's a Class C network.
Numbers above 223 are reserved for other purposes. The smaller
the netid, the fewer number of networks that can be subnetted,
but the larger number of hosts on the network. A Class A rating
is best for large networks while a Class C is best for small ones.
To create a subnet, the demarcation line on the IP address is
moved between the netid and the hostid, to give the netid more
bits to work with and to take away bits from the hostid. To do
this, a special number called a subnet mask is used.
Subnetting is used when intranets grow over a certain size and
they begin to have problems. One problem is management of host
IP addresses-making sure that every computer on the network has
a proper, up-to-date host address, and that old host addresses
are put out of use until needed in the future. In a corporation
spread out over several locations-or across the world-it's difficult,
if not impossible, to have one person responsible for managing
the host addresses at every location and department in the company.
Another problem has to do with a variety of hardware limitations
of networks. Dissimilar networks may all be part of an intranet.
An intranet may have some sections that are Ethernet, other sections
that are Token Ring networks, and conceivably other sections that
use different networking technologies altogether. There is no
easy way for an intranet router to link these dissimilar networks
together and route the information to the proper places.
Another set of problems has to do with the physical limitations
of network technology. In some kinds of networks, there are some
strict limitations on how far cables can extend in the network.
In other words, you can't go over a certain distance of cabling
without using repeaters or routers. A "thick" Ethernet
cable, for example, can only be extended to 500 meters, while
a "thin" Ethernet cable can only go to 300 meters. Routers
can be used to link these cables together, so that an intranet
can be extended well beyond those distances. But when that is
done, each length of wire is essentially considered its own subnetwork.
Yet one more set of problems has to do with the volume of traffic
that travels across an intranet. Often in a corporation, in a
given department, most of the traffic is intradepartmental traffic-in
other words, mail and other data that people within a department
send to each another. The volume of traffic outside to other departments
is considerably less. What's called for is a way to confine intradepartmental
traffic inside the departments, to cut down on the amount of data
that needs to be routed and managed across the entire intranet.
Subnetting solves all these problems and more. When an intranet
is divided into subnets, one central administrator doesn't have
to manage every aspect of the entire intranet. Instead, each subnet
can take care of its own administration. That means smaller organizations
within the larger organization can take care of problems such
as address management and a variety of troubleshooting chores.
If an intranet is subnetted by divisions or departments, it means
that each division or department can guide the development of
its own network, while adhering to general intranet architecture.
Doing this allows departments or divisions more freedom to use
technology to pursue their business goals.
Subnets also get around problems that arise when an intranet has
within it different kinds of network architecture, such as Ethernet
and Token Ring technologies. Normally-if there is no subnetting-a
router can't link these different networks together because they
don't have their own addresses. However, if each of the different
networks is its own subnet-and so has its own network address-routers
can then link them together and properly route intranet traffic.
Subnetting can also cut down on the traffic traveling across the
intranet and its routers. Since much network traffic may be confined
within departments, having each department be its own subnet means
that all that traffic need never cross an intranet router and
cross the intranet-it will stay within its own subnet.
Subnetting can also increase the security on an intranet. If the
payroll department, for example, were on its own subnet, then
much of its traffic would not have to travel across an intranet.
Having its data traveling across the intranet could mean that
someone could conceivably hack into the data to read it. Confining
the data to its own subnet makes that much less likely to happen.
Dividing an intranet into subnets can also make the entire intranet
more stable. If an intranet is divided in this way, then if one
subnet goes down or is often unstable, it won't affect the rest
of the intranet.
This all may sound rather confusing. To see how it's done, let's
take a look at a network, and see how to use the IP address to
create subnets. Let's say we have a Class B network. That network
is assigned the address of 130.97.0.0. When a network is given
an address, it is assigned the netid numbers-in this case, the
130.97-and it can assign the host numbers (in this case, 0.0)
in any way that it chooses.
The 130.97.0.0 network is a single intranet. It's getting too
large to manage, though, and we've decided to divide it into two
subnets. What we do is fairly straightforward. We take a number
from the hostid field and use it to identify each of the subnets.
So one subnet gets the address 130.97.1.0, and the other gets
the address 130.97.2.0. Individual machines on the first subnet
get addresses of 130.97.1.1, 130.97.1.2, and so on. Individual
machines on the second subnet get addresses of 130.97.2.1, 130.97.2.2
and so on.
Sounds simple. But we have a problem. The Internet doesn't recognize
130.97.1.0 and 130.97.2.0 as separate networks. It treats them
both as 130.97.0.0 since the "1" and "2" that
we're using as a netid is only known to the Internet as a hostid.
So our intranet router will not be able to route incoming traffic
to the proper network.
To solve the problem, a subnet mask is used. A subnet mask is
a 32-bit number in IP form used by intranet routers and hosts
that will help routers understand how to route information to
the proper subnet. To the outside Internet, there is still only
one network, but the subnet mask allows routers inside the intranet
to send traffic to the proper host.
A subnet mask is a number such as 255.255.255.0 (the built-in
default for Class C addresses; the Class B default is 255.255.0.0
and the default for Class A is 255.0.0.0). A router takes the
subnet mask and applies that number against the IP number of incoming
mail to the network by using it to perform a calculation. Based
on the resulting IP number, it will route mail to the proper subnet,
and then to a particular computer on the subnet. For consistency,
everyone in a particular intranet will use the same subnet mask.
When intranets are over a certain size, or are spread over several
geographical locations, it becomes difficult to manage them as
a single network. To solve the problem, the single intranet can
be subdivided into several subnets, subsections of an intranet
that make them easier to manage. To the outside world, the intranet
still looks as if it's a single network.
- If you're building an intranet and want it to be connected
to the Internet, you'll need a unique IP address for your intranet
network, which the InterNIC Registration Services will handle.
There are three classes of intranet you can have: Class A, Class
B, or Class C. Generally, a Class A rating is best for the largest
networks, while a Class C is best for the smallest. A Class A
network can be composed of 127 networks, and a total of 16,777,214
nodes on the network. A Class B network can be composed of 16,383
networks, and a total of 65,534 nodes. A Class C network can be
composed of 2,097,151 networks, and 254 nodes.
- When an intranet is assigned an address, it is assigned the
first two IP numbers of the Internet numeric address (called the
netid field) and the remaining two numbers (called the
hostid field) are left blank, so that the intranet itself
can assign them, such as 147.106.0.0. The hostid field consists
of a number for a subnet and a host number.
- When an intranet is connected to the Internet, a router handles
the job of sending packets into the intranet from the Internet.
In our example, all incoming mail and data comes to a router for
a network with the netid of 147.106.0.0.
- When intranets grow-for example, if there is a department
located in another building, city, or country-there needs to be
some way to manage network traffic. It may be impractical and
physically impossible to route all the data necessary among many
different computers spread across a building or the world. A second
network-called a subnetwork or subnet-needs to be
created.
- In order to have a router handle all incoming traffic for
a subnetted intranet, the first byte of the hostid field is used.
The bits that are used to distinguish among subnets are called
subnet numbers. In our example, there are two subnets on
the intranet. To the outside world, there appears to be only one
network.
- Each computer on each subnet gets its own IP address, as in
a normal intranet. The combination of the netid field, the subnet
number, and then finally a host number, forms the IP address.
- The router must be informed that the hostid field in subnets
must be treated differently than non-subnetted hostid fields,
otherwise it won't be able to properly route data. In order to
do this, a subnet mask is used. A subnet mask is a 32-bit
number such as 255.255.0.0 that is used in concert with the numbers
in the hostid field. When a calculation is performed using the
subnet mask and the IP address, the router knows where to route
the mail. The subnet mask is put in people's network configuration
files.
Contact
reference@developer.com with questions or comments.
Copyright 1998
EarthWeb Inc., All rights reserved.
PLEASE READ THE ACCEPTABLE USAGE STATEMENT.
Copyright 1998 Macmillan Computer Publishing. All rights reserved.
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