Curs PC si internet cap 10.1 Importance of a Network Layer
Overview The network layer is responsible for navigating the data through the network. The function of the network layer is to find the best path through the network. The network layer's addressing scheme is used by devices to determine the destination of data as it moves through the network. In this chapter, you will learn about the routers use and operations in performing the key internetworking function of the Open System Interconnection (OSI) reference models network layer, Layer 3.In addition, you will learn about IP addressing and the three classes of networks in IP addressing schemes. You also will learn that some IP addresses have been set aside by the American Registry for Internet Numbers (ARIN) and cannot be assigned to any network. Finally, you will learn about subnetworks and subnet masks and their IP addressing schemes. 10.1 Importance of a Network Layer 10.1.1 Identifiers Instructor Note The purpose of this target indicator is to justify the necessity of Layer 3 addresses. The key distinction to make for the students is that MAC addresses represent a flat address space. That is, they are non-hierarchical like national personal identification (e.g., social security) numbers. MAC addressing -- the naming of computers with hexadecimal numbers -- works fine in a LAN environment, but they don't scale well. As the number of computers and separate networks grows, the necessity of some kind of hierarchical addressing scheme becomes apparent. Telephone and Postal codes are routing codes which are analogous to layer 3 addressing schemes. As an activity you might have the students drawing a diagram for n = 30 computers might help. Label them A, B, C, etc. and then relabel and reorganize the computers hierarchically with two-part numerical codes. Discuss the implications of both addressing schemes. The network layer is responsible for moving data through a set of networks (internetwork). The network layer's addressing scheme is used by devices to determine the destination of data as it moves through the networks.Protocols that have no network layer can only be used on small internal networks. These protocols usually use only a name (i.e. MAC address) to identify the computer on a network. The problem with this approach is that, as the network grows in size, it becomes increasingly difficult to organize all the names, such as making sure that two computers aren't using the same name. Protocols that support the network layer use an identification technique for devices that guarantees a unique identifier. So how does this identifier differ from a MAC address, which is also unique? MAC addresses use a flat addressing scheme that makes it difficult to locate devices on other networks. Network layer addresses use a hierarchical addressing scheme that allows for unique addresses across network boundaries, along with a method for finding a path for data to travel between networks. Hierarchical addressing schemes enable information to traverse an internetwork, along with a method to find the destination in an efficient fashion. The telephone network is an example of the use of hierarchical addressing. The telephone system uses an area code that designates a geographical area for the call's first stop (hop). The next three digits represent the local exchange (second hop). The final digits represent the individual destination telephone (which is, or course, the final hop). Network devices need an addressing scheme that allows them to forward data packets through the internetwork (a set of networks composed of multiple segments using the same type of addressing). There are several network layer protocols with different addressing schemes that allow devices to forward data throughout an internetwork. Web Links RFC 941 Routing and the Network Layer Network Layer Functions 10.1 Importance of a Network Layer 10.1.2 Segmentation and autonomous systems Instructor Note There are two main points to this target indicator. First, that multiple networks are desirable (we create them to segment our networks into smaller networks for traffic management) and that multiple networks already exist (the Internet is a WAN comprised of millions of smaller networks all of which want to be somewhat connected). Secondly, this target indicator makes use of the highway analogy for networking. This analogy was introduced in Chapter 1 and is a rich analogy for many aspects of networking. Particularly important is that routing takes place in highway systems (perhaps have the students brainstorm how this occurs -- i.e., maps, traffic signs and signals, people getting directions, etc) and that large data networks need routing information as well. There are two primary reasons why multiple networks are necessary - the growth in size of each network and the growth in the number of networks. When a LAN, MAN, or WAN grows, it may become necessary or desirable for network traffic control to break it up into smaller pieces called network segments (or just segments). This results in the network becoming a group of networks, each requiring a separate address. There are already a vast number of networks in existence; separate computer networks are common in offices, schools, companies, businesses, and countries. While it is convenient to have these separate networks (or autonomous systems, if each is managed by a single administration) communicate with each other over the Internet, they must do it with sensible addressing schemes and appropriate internetworking devices. If not, the network traffic flow would become severely clogged, and neither the local networks, nor the Internet, would function.An analogy that might help you understand the need for network segmentation is to imagine a highway system and the number of vehicles that use it. As the population in the areas surrounding the highways increases, the roads become burdened with too many vehicles. Networks operate much in the same way. As networks grow, the amount of traffic grows. One solution might be to increase the bandwidth, much the same as increasing the speed limits of, or adding lanes to, the highways. Another solution might be to use devices that segment the network and control the flow of traffic, the same way a highway would use devices such as stoplights to control the movement of traffic. 10.1 Importance of a Network Layer 10.1.3 Communication between separate networks Instructor Note The importance of this target indicator can be rephrased as "why would we want to have an Internet." The world is just beginning to answer this question; every day some new purpose is found for the world-wide interconnection of networks known as the Internet. The knowledge sharing, the commerce, the near instantaneous personalized communications, and many other reasons are why separate networks would "need" to communicate. Perhaps you can challenge members of your class to come up with new ways to use the Internet! The Internet is a collection of network segments that are tied together to facilitate the sharing of information. Once again, a good analogy would be the example of the highway system with the large multiple lanes that have been constructed to interconnect many geographical regions. Networks operate in much the same manner, with companies known as Internet service providers (ISPs) offering services that tie together multiple network segments. 10.1 Importance of a Network Layer 10.1.4 Layer 3 network devices Instructor Note There are three key points to this target indicator: routers connect separate networks, routers make best path decisions based on layer 3 information, and routers actually switch packets from incoming ports to appropriate outgoing ports. You cannot stress these three points enough -- everything that follows in Chapters 10 and 11 is in some way justified so the router can perform one of these functions. Without routers you could not connect separate networks efficiently, there would be no devices intelligent enough to route packets along a best path nor to switch them to that best path. Internetworking devices that operate at OSI Layer 3 (the network layer) tie together, or interconnect, network segments or entire networks. These devices are called routers. They pass data packets between networks based on network protocol, or Layer 3, information. Routers make logical decisions regarding the best path for the delivery of data on an internetwork and then direct packets to the appropriate output port and segment. Routers take packets from LAN devices (e.g. workstations) and, based on Layer 3 information, forward them through the network. In fact, routing is sometimes referred to as Layer 3 switching.
