Lesson 1: Using the Network And Sharing Center to Configure Networking

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So what identifies the computer and what identifies the subnet? To discover this we need to

look at the next value, the subnet mask.

Subnet masks are most peculiar numbers. They represent binary numbers that consist of all

ones followed by all zeros. For example:

255.255.255.0 is the binary number 11111111 11111111 11111111 00000000.

The actual value of this number is irrelevant. What matters is the number of ones and zeros.

A one says that the corresponding bit in the IPv4 address is a network address bit. A zero says

that the corresponding bit in the IPv4 address is a computer or host address bit.

In the example given, the last eight bits of the subnet mask are all zero. So, the host address is

the final octet of the subnet address, or 143. The network address of the subnet is 10.16.10.0.

Because hosts are defined by a single octet in this example, the 10.16.10.0 subnet contains 254

host addresses. The first IPv4 address in the subnet is 10.16.10.1. The last is 10.16.10.254. The

number 10.16.10.0 identifies the subnet and is called the subnet address. The number

10.16.10.255 is called the broadcast address and is used when a datagram needs to be sent to

every host on a subnet.

Subnetting and Supernetting

You can split a subnet into smaller subnets by adding ones to the end of the ones in the

subnet mask. If you have two (or more) suitable contiguous subnets, you can merge them

into a single subnet by changing one or more ones at the end of the ones in the subnet

masks to zeros. These techniques are known as subnetting and supernetting, respectively.

If an organization has a significant number of computers on its network (say more than

100—this number varies depending on the type, volume, and pattern of traffic on the net-

work) or if it has several geographic locations, the organization will probably create sev-

eral subnets. If a subnet contains too many computers and other devices, it tends to slow

down because there is a greater chance of two computers trying to put data onto the net-

work simultaneously and causing a collision. Dividing a network into several subnets

reduces the likelihood of such collisions.

At the router that connects to the Internet, however, the organization will use supernet-

ting to combine (or summarize) the subnets so that they can be defined with a single

network address that will be translated to a public address on the Internet. Public

addresses and address translation are discussed later in this lesson.

Supernetting, subnetting, and the related classless interdomain routing (CIDR) and vari-

able length subnet mask (VLSM) technologies are unlikely to be tested in the 70-620

examination. Nevertheless, if you want to find out more (and I recommend that you do),

access http://www.howtosubnet.com and http://support.microsoft.com/kb/164015.