10.2 Path Determination 10.2.1 Path determination Instructor Note Of course you can make more complicated kinesthetic activities with multiple students acting as multiple routers with multiple paths, but the highway analogy is probably better. In other words, pose the problem to the students -- how do we get from point A to point B in a city at rush hour when there's been an accident on the main highway? This will illustrate the notion of best path selection. Having a map of the city and having the students choose best paths is a simple and illustrative activity. You can then compare this to routing processes. Having students discuss best paths simulates routing protocols (about which they will learn later). Again, the idea is to make as many of the abstractions as tangible as possible. Path determination occurs at Layer 3 (network layer), and enables a router to evaluate the available paths to a destination, and to establish the preferred handling of a packet. Routing services use network topology information when evaluating network paths. Path determination is the process that the router uses to choose the next hop in the path for the packet to travel to its destination. This process is also called routing the packet.Path determination for a packet can be compared to a person driving a car from one side of a city to the other. The driver has a map that shows the streets that he/she needs to take to get to the destination. The drive from one intersection to another is a hop. Similarly, a router uses a map that shows the available paths to a destination. Routers can also make their decisions based on the traffic density and the speed of the link (bandwidth), just as a driver may choose a faster path (a highway) or use less crowded back streets. Web Links Routers 10.2 Path Determination 10.2.2 Network layer addressing Instructor Note We have made the distinction between "naming" a computer with a MAC address and "addressing" a computer with a network layer address. This target indicator strives to emphasize the difference. You might pose the problem to the students -- would routing be possible if we just had names (MAC addresses) for computers? What problems would arise and what would such Layer 2 "routing" devices have to look like (amongst other problems they would have to remember the name of every single device on all networks in order to route any information, hence the layer 2 routing tables would be ridiculously large). Then emphasize how hierarchical addressing, when combined with naming, gives us efficient local delivery but also efficient world-wide routing and delivery of information. The network address helps the router identify a path within the network cloud. The router uses the network address to identify the destination network of a packet within an internetwork. For some network layer protocols, a network administrator assigns network addresses according to some predetermined internetwork addressing plan. For other network layer protocols, assigning addresses is partially or completely dynamic/automatic. In addition to the network address, network protocols use some form of host, or node, address. The graphic shows three devices in Network 1 (two workstations and a router), each with its own unique host address. (it also shows that the router is connected to two other networks - Networks 2 & 3).Addressing occurs at the network layer. Earlier analogies of a network address include the first portions (area code and first three digits) of a telephone number. The remaining (last four) digits of a phone number, which tell the phone company equipment which specific phone to ring, are like the host portion of an address, which tells a router to which specific device it should deliver a packet.Without network layer addressing, routing can not take place. Routers require network addresses to ensure proper delivery of packets. Without some hierarchical addressing structure, packets would not be able to travel across an internetwork. In a similar way, without some hierarchical structure to telephone numbers, postal addresses, or transportation systems, there would not be a smooth delivery of the goods and services. Web Links Routers 10.2 Path Determination 10.2.3 Layer 3 and computer mobility Instructor Note The purpose of this target indicator is to emphasize another benefit of a two-tiered, hierarchical addressing scheme: computers can be moved and the network can accommodate moves with a minimum of disruption. Computers keep their name (their MAC address) but can change their address (their network layer address). A MAC address can be compared to your name and the network address to your mailing 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 physically move a computer to a different network, the computer maintains the same MAC address, but you must assign it a new network address. Web Links Routers 10.2 Path Determination 10.2.4 Comparing flat and hierarchical addressing Instructor Note The purpose of this target indicator is both summary and introduction. Flat and hierarchical addressing schemes have been extensively mentioned in prior target indicators. So this summarizes the main points of those target indicators. But a grand introduction is made: the network layer addressing scheme, the layer 3 protocol to be used in the class -- Internet Protocol, or IP -- is introduced. IP addressing is one of the most important topics throughout all four semesters of the curriculum and on the CCNA exam. The function of the network layer is to find the best path through the network. To accomplish this, it uses two addressing methods - flat addressing and hierarchical addressing. A flat addressing scheme assigns a device the next available address. There is no thought given to the structure of the addressing scheme. An example of a flat addressing scheme would be military identification numbering system, or a birth identification numbering system. MAC addresses function in the same manner. A vendor is given a block of addresses; the first half of each address is for the vendor's code, the rest of the MAC address is a number that has been sequentially assigned.In a hierarchical addressing scheme, such as that used by the postal system ZIP Codes, the address is determined by the location of the building, not by a randomly assigned number. The addressing scheme that you will use throughout this course is Internet Protocol (IP) addressing. IP addresses have a specific structure and are not randomly assigned. Web Links Routers
