Chapter 4 - The network layer is responsible for...

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The network layer is responsible for transferring packets of data from the source computer to the destination computer via one or more networks. The data-link layer is responsible for transferring data within a network. In general, though, the source and destination of a data packet can be located on different networks. For example, when you visit your school's home page from your home PC, the source is on your home network and the destination is on the university network. The data-link layer cannot exchange data between these two computers, and the network layer becomes necessary to transfer data across networks. This function of the network layer may be summarized in one word —routing. Routing is the process of selecting a path on the Internet which can be used to deliver data to a destination. Routing is performed in networks by devices called routers . Routers are devices that connect two or more networks and forward incoming packets to the appropriate connected network. Routing is examined in Chapter 8. This chapter focuses on the tasks performed by the source and destination computers to help routers perform their routing function. Figure 4-1 provides an overview of the role of routers. When a network grows to the point where it is too large for an Ethernet, the network can be divided into multiple Ethernets. A router can be used to connect the Ethernets, creating a network of networks. To help routers perform their routing function, every device on the network is assigned a unique network layer identifier. Since this address is defined by IP, the network layer protocol, this address is popularly called the IP address of the computer. IP addresses are identifiers used as addresses of computer resources on the Internet. Before sending a data packet out on the network, the sender adds the destination's IP address to the header of every data packet. In Chapter 3, we saw that networked computers also have MAC addresses. You might wonder how data-link layer addresses are related to IP addresses. If you simply want to identify a computer on the network, why do you need two different addresses for every computer? The relationship between data-link layer and IP addresses is clarified in Figure 4-2. The path from source to destination comprises many hops. A hop is a link from a router to the next neighboring router. The data- link layer address allows the packet to be delivered correctly within each individual network (i.e. over one hop) on the path to the destination. IP address retains the address of the actual source and the final destination. Note in Figure 4-2 that the destination IP address docs not change throughout the packet's journey from the source (PC) to the destination (web server). However, the data-link layer addresses keep changing as the packet gets transferred from network to network. Thus, immediately as the packet leaves the PC, the data-link destination address points the packet to the nearest router. The data-link layer technology on this hop uses the data-link layer destination address to deliver the packet to router l. the 2 hop, the data-
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  • Fall '08
  • Staff
  • IP address, IP addresses

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