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.

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.

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 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.

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.

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. 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.

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.

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.

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.

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.

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.

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.

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.

In addition to the network address, network protocols use some form of host, or node, address. For some network layer protocols, a network administrator assigns network host addresses according to some predetermined internetwork addressing plan. For other network layer protocols, assigning host addresses is partially or completely dynamic/automatic. 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 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.

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.

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.

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.

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.

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). 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.

Figures and explain this further. For more information on IP and IP addressing, visit some of these sites:

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)
• 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)
• fragment offset - the field that is used to help piece together datagram fragments (13 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)
• 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

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.

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 ...

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.

What is the decimal value of the highlighted octet in the graphic? What is the value of the bit on the far left side? The next bit? Since those are the only 2 bits on (or set), then the decimal value is 128+64=192!

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.

There are three classes of IP addresses that an organization can receive from the American Registry for Internet Numbers (ARIN) (or the organization's ISP). They are Class A, B, and C. ARIN now reserves Class A addresses for governments throughout the world (although a few large companies, such as Hewlett Packard, have received one in the past) and Class B addresses for medium-sized companies. All other requestors are issued Class C addresses. 

IP addresses identify a device on a network and the network to which it is attached. To make them easy to remember, IP addresses are usually written in dotted decimal notation. Therefore, IP addresses are 4 decimal numbers separated by dots. An example of this is the address 166.122.23.130. Keep in mind that a decimal number is a base 10 number, the type we use in everyday life.

Each place in an octet represents a different power of 2. As in the Base 10 number system, the powers increase from right to left.

Figure   illustrates a method for converting binary numbers to decimal numbers. Figure allows you to practice your conversion skills.

Figure   illustrates a method for converting decimal numbers to binary numbers. Figure allows you to practice your conversion skills.

Example:
10010000 (Work from right to left).

In this example, there are 0 values of 2⁰; 0 values of 2¹; 0 values of 2²; 0 values of 2³; 1 value of 2⁴; 0 values of 2⁵; 0 values of 2⁶; and 1 value of 2⁷. There are no 1s, no 2s, no 4s, no 8s, one 16s, no 32s, no 64, and one 128. Added together, the values total 144, therefore, the binary number 10010000 equals the decimal number 144.

In order to convert decimal IP addresses to binary numbers you must know the decimal values of each of the 8 bits in each octet. Starting with the bit that is on the left side of the octet, the values start at 128 and are reduced by half each time you move 1 bit to the right, continuing to a value of 1 on the right side of the octet.. The conversion below illustrates the first octet only.

Example:
Convert the first octet of 192.57.30.224 to a binary format.

The first step is to select the octet on the far left and determine whether the value is greater than 128. In this instance (192), it is. Then place a 1 in the first bit and subtract 128 from 192. The remainder is 64. The value of the next bit is 64, which is equal to the value of the remainder, so that bit would be 1 as well. Subtract 64 from 64. The remainder is 0, therefore the remaining bits would all be 0. The binary number for the first octet would be 11000000.

Exercise:
Convert the remaining octets (57, 30, 224), in the IP address, to binary format.

To convert binary IP addresses to decimal numbers, use the opposite approach you used to convert decimal numbers to binary numbers.

Example:
Convert the first octet of the binary IP address 10101010.11111111.00000000.11001101 to a dotted decimal number.

To convert this IP address, start with the bit that is on the far left side in the first octet. It is 1. You know that the value of a bit in that position is 128, therefore the decimal number starts with a value of 128. The next value is 0, so skip it. The third value is 1; any bit in that position has a value of 32; therefore, you add 32 to 128 to get 160. The fourth bit is 0, so skip it. The fifth bit is 1, which means that you add 8 to the current total of 160, giving you a new total of 168. The sixth bit is also 0, so skip it and the seventh bit is 1, which means add 2 to the current total of 168. The last bit is 0, so you can skip it.