1. Network Layer

2. 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 router’s use and operations in performing the key internetworking function of the Open System Interconnection (OSI) reference model’s network layer, Layer 3.

3. Overview 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.

4. Importance of a Network Layer : Identifiers 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.

5. Importance of a Network Layer : Identifiers 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.

6. Importance of a Network Layer : Identifiers Protocols that support the network layer 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. While MAC addresses use a flat addressing scheme that makes it difficult to locate devices on other networks.

7. Importance of a Network Layer : Identifiers 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).

8. Importance of a Network Layer : Identifiers 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.

9. Importance of a Network Layer : Segmentation and autonomous systems 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.

10. Importance of a Network Layer : Segmentation and autonomous systems There are already a vast number of networks in existence; separate computer networks are common in offices, schools, companies, businesses, and countries 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. However, 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.

11. Importance of a Network Layer : Segmentation and autonomous systems 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.

12. Importance of a Network Layer : Segmentation and autonomous systems

13. Importance of a Network Layer : Communication between separate networks 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.

14. Importance of a Network Layer : Communication between separate networks Networks operate in much the same manner, with companies known as Internet service providers (ISPs) offering services that tie together multiple network segments.

15. Importance of a Network Layer : Layer 3 network devices Routers are internetworking devices which operate at OSI layer 3 (the network layer). They tie together, or interconnect, network segments or entire networks. They pass data packets between networks based on Layer 3 information

16. Importance of a Network Layer : Layer 3 network devices 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

17. Path Determination Path determination occurs at Layer 3 (network layer). It 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.

18. Path Determination

19. 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.

20. Path Determination 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.

21. Path Determination : Network layer addressing 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.

22. Path Determination : Network layer addressing 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).

23. Path Determination : Network layer addressing

24. 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 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 to which specific device it should deliver a packet.

25. Path Determination : Network layer addressing 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.

26. Path Determination : Layer 3 and computer mobility 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.

27. Path Determination : Comparing flat and hierarchical addressing 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.

28. Path Determination : Comparing flat and hierarchical addressing 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.

29. Path Determination : Comparing flat and hierarchical addressing The postal system ZIP codes are a good example of hierarchical addressing. In the ZIP code system 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.

30. Path Determination : Comparing flat and hierarchical addressing

31. IP Addresses within the IP Header : Network layer datagrams The Internet Protocol (IP) is the most popular implementation of a hierarchical network addressing scheme. IP is the network protocol the Internet uses. As information flows down the layers of the OSI model, the data is encapsulated at each layer. At the network layer, the data is encapsulated within packets (also known as datagrams).

32. IP Addresses within the IP Header : Network layer datagrams IP determines the form of the IP packet header (which includes addressing and other control information), but does not concern itself with the actual data -- it accepts whatever is passed down from the higher layers.

33. IP Addresses within the IP Header : Network layer datagrams

34. IP Addresses within the IP Header : Network Layer Fields

35. The Layer 3 packet/datagram becomes the Layer 2 data, which is then encapsulated into frames (as previously discussed). Similarly, the IP packet consists of the data from upper layers plus an IP header, which consists of: – version - indicates the version of IP currently used (4 bits) – IP header length (HLEN) - indicates the datagram header length in 32 bit words (4 bits)

36. IP Addresses within the IP Header : Network Layer Fields – type-of-service - specifies the level of importance that has been assigned by a particular upper-layer protocol (8 bits) – total length - specifies the length of the entire IP packet, including data and header, in bytes (16 bits) – identification - contains an integer that identifies the current datagram (16 bits) – flags - a 3-bit field in which the 2 low-order bits control fragmentation – one bit specifying whether the packet can be fragmented, and the second whether the packet is the last fragment in a series of fragmented packets (3 bits)

37. IP Addresses within the IP Header : Network Layer Fields – fragment offset - the field that is used to help piece together datagram fragments (16 bits) – time-to-live - maintains a counter that gradually decreases, by increments, to zero, at which point the datagram is discarded, keeping the packets from looping endlessly (8 bits) – protocol - indicates which upper-layer protocol receives incoming packets after IP processing has been completed (8 bits) – header checksum - helps ensure IP header integrity (16 bits)

38. IP Addresses within the IP Header : Network Layer Fields – source address - specifies the sending node (32 bits) – destination address - specifies the receiving node (32 bits) options - allows IP to support various options, such as security (variable length) – data - contains upper-layer information (variable length, maximum 64 Kb) – padding - extra zeros are added to this field to ensure that the IP header is always a multiple of 32 bits

39. IP Addresses within the IP Header: IP header source and destination fields The IP address contains the information that is necessary to route a packet through the network. Each source and destination address field contains a 32 bit address. The source address field contains the IP address of the device that sends the packet. The destination field contains the IP address of the device that receives the packet.

40. IP Addresses within the IP Header: IP header source and destination fields

41. IP Addresses within the IP Header : IP address as a 32-bit binary number An IP address is represented by a 32 bit binary number. As a quick review, remember that each binary digit can be only 0 or 1. In a binary number, the value of the right-most bit (also called the least significant bit) is either 0 or 1. The corresponding decimal value of each bit doubles as you move left in the binary number. So the decimal value of the 2 nd bit from the right is either 0 or 2. The third bit is either 0 or 4, the fourth bit 0 or 8, etc...

42. IP Addresses within the IP Header : IP address as a 32-bit binary number

43. IP addresses are expressed as dotted-decimal numbers - we break up the 32 bits of the address into four octets (an octet is a group of 8 bits). The maximum decimal value of each octet is 255. The largest 8 bit binary number is 11111111. Those bits, from left to right, have decimal values of 128, 64, 32, 16, 8, 4, 2, and 1. Added together, they total 255.

44. IP Addresses within the IP Header: IP address component fields The network number of an IP address identifies the network to which a device is attached. The host portion of an IP address identifies the specific device on that network. Because IP addresses consist of four octets separated by dots, one, two, or three of these octets may be used to identify the network number. Similarly, up to three of these octets may be used to identify the host portion of an IP address.