Page 111 - Handout Computer Network.

its own unique host address. (The figure also shows that the router is connected to two other
networks—networks 2 and 3.)

Figure 5-4 shown a Router Interfaces Connection

Logical addressing occurs at the network layer. Recall the analogy that compares network
addresses to telephone numbers. The first portions of a phone number are the area code and the first
three digits. The last four digits of a phone number tell the phone company equipment which specific
phone to ring. This is similar to the function of the host portion of an address.
The host portion tells the router the specific device to which it should deliver a packet. Without
network layer addressing, routing cannot take place. Routers require network addresses to ensure
proper delivery of packets. Without some hierarchical addressing structure, packets could not travel
across an internetwork.
Similarly, without some hierarchical structure to telephone numbers, postal addresses, or
transportation systems, there would be no smooth delivery of goods and services.

A MAC address can be compared to your name, and a network layer address can be compared
to your mailing address (network and host address). For example, if you were to move to another
town, your name would remain unchanged, but your mailing address would indicate your new
location. Network devices (routers as well as individual computers) have both a MAC address and a
protocol (network layer) address.

When you move a computer to a different network, the computer maintains the same MAC
address, but you must assign it a new network layer address.

5.3.3 The Communication Path

The function of the network layer is to find the best path through the network. To be truly
practical, a network must consistently represent the paths available between routers.

As Figure 5-5 shows, each line between the routers has a number that the routers use as a
network address. These addresses must convey information that can be used by a routing process.