TAILIEUCHUNG - CCNA 1 and 2 Companion Guide, Revised (Cisco Networking Academy Program) part 48

Cisco Networking Academy Program CCNA 1 and 2 Companion Guide, Revised part 48 is the Cisco approved textbook to use alongside version of the Cisco Networking Academy Program CCNA 1 and CCNA 2 web-based courses. The topics covered provide you with the necessary knowledge to begin your preparation for the CCNA certification exam (640-801, or 640-821 and 640-811) and to enter the field of network administration. | Page 439 Tuesday May 20 2003 2 53 PM Routed Routable and Routing Protocols 439 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. 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 8-13 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. This means that an address must have information about the path of media connections that the routing process uses to pass .

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