ADDRESSING

Four levels of addresses are used in an internet employing the TCP/IP protocols: physical (link) addresses, logical (IP) addresses, port addresses, and specific addresses (see Figure).

Physical Addresses

The physical address, also known as the link address, is the address of a node as defined by its LAN or WAN. It is included in the frame used by the data link layer. It is the lowest-level address.

The physical addresses have authority over the network (LAN or WAN). The size and format of these addresses vary depending on the network. For example, Ethernet uses a 6-byte (48-bit) physical address that is imprinted on the network interface card (NIC). LocalTalk (Apple), however, has a I-byte dynamic address that changes each time the station comes up.

Example 2.1

In Figure 2.19 a node with physical address 10 sends a frame to a node with physical address 87.

The two nodes are connected by a link (bus topology LAN). At the data link layer, this frame

contains physical (link) addresses in the header. These are the only addresses needed. The rest of

the header contains other information needed at this level. The trailer usually contains extra bits

needed for error detection. As the figure shows, the computer with physical address lOis the

sender, and the computer with physical address 87 is the receiver. The data link layer at the

sender receives data from an upper layer. It encapsulates the data in a frame, adding a header and

a trailer. The header, among other pieces of information, carries the receiver and the sender physical

(link) addresses. Note that in most data link protocols, the destination address, 87 in this

case, comes before the source address (10 in this case).

We have shown a bus topology for an isolated LAN. In a bus topology, the frame is propagated

in both directions (left and right). The frame propagated to the left dies when it reaches the

end of the cable if the cable end is terminated appropriately. The frame propagated to the right is

sent to every station on the network. Each station with a physical addresses other than 87 drops

the frame because the destination address in the frame does not match its own physical address.

The intended destination computer, however, finds a match between the destination address in the

frame and its own physical address. The frame is checked, the header and trailer are dropped, and

the data part is decapsulated and delivered to the upper layer.

Example 2.2

As we will see in Chapter 13, most local-area networks use a 48-bit (6-byte) physical address

written as 12 hexadecimal digits; every byte (2 hexadecimal digits) is separated by a colon, as

shown below:

07:01:02:01 :2C:4B

A 6-byte (12 hexadecimal digits) physical address

Logical Addresses

Logical addresses are necessary for universal communications that are independent of

underlying physical networks. Physical addresses are not adequate in an internetwork

environment where different networks can have different address formats. A universal

addressing system is needed in which each host can be identified uniquely, regardless

of the underlying physical network.

The logical addresses are designed for this purpose. A logical address in the Internet

is currently a 32-bit address that can uniquely define a host connected to the Internet. No

two publicly addressed and visible hosts on the Internet can have the same IP address.

Example 2.3

Figure 2.20 shows a part of an internet with two routers connecting three LANs. Each device

(computer or router) has a pair of addresses (logical and physical) for each connection. In this

case, each computer is connected to only one link and therefore has only one pair of addresses.

Each router, however, is connected to three networks (only two are shown in the figure). So each

router has three pairs of addresses, one for each connection. Although it may obvious that each

router must have a separate physical address for each connection, it may not be obvious why it

needs a logical address for each connection. We discuss these issues in Chapter 22 when we discuss

routing.

The computer with logical address A and physical address 10 needs to send a

packet to the computer with logical address P and physical address 95. We use letters to

show the logical addresses and numbers for physical addresses, but note that both are

actually numbers, as we will see later in the chapter.

The sender encapsulates its data in a packet at the network layer and adds two logical

addresses (A and P). Note that in most protocols, the logical source address comes before

the logical destination address (contrary to the order of physical addresses). The network

layer, however, needs to find the physical address of the next hop before the packet can be

delivered. The network layer consults its routing table (see Chapter 22) and finds the

logical address of the next hop (router I) to be F. The ARP discussed previously finds

the physical address of router 1 that corresponds to the logical address of 20. Now the

network layer passes this address to the data link layer, which in tum, encapsulates the

packet with physical destination address 20 and physical source address 10.

The frame is received by every device on LAN 1, but is discarded by all except

router 1, which finds that the destination physical address in the frame matches with its

own physical address. The router decapsulates the packet from the frame to read the logical

destination address P. Since the logical destination address does not match the

router's logical address, the router knows that the packet needs to be forwarded. The

router consults its routing table and ARP to find the physical destination address of the

next hop (router 2), creates a new frame, encapsulates the packet, and sends it to router 2.

Note the physical addresses in the frame. The source physical address changes

from 10 to 99. The destination physical address changes from 20 (router 1 physical

address) to 33 (router 2 physical address). The logical source and destination addresses

must remain the same; otherwise the packet will be lost.

At router 2 we have a similar scenario. The physical addresses are changed, and a

new frame is sent to the destination computer. When the frame reaches the destination,

the packet is decapsulated. The destination logical address P matches the logical address

of the computer. The data are decapsulated from the packet and delivered to the upper

layer. Note that although physical addresses will change from hop to hop, logical

addresses remain the same from the source to destination. There are some exceptions to

this rule that we discover later in the book.

The physical addresses will change from hop to hop,

but the logical addresses usually remain the same.

Port Addresses

The IP address and the physical address are necessary for a quantity of data to travel

from a source to the destination host. However, arrival at the destination host is not the

final objective of data communications on the Internet. A system that sends nothing but

data from one computer to another is not complete. Today, computers are devices that

can run multiple processes at the same time. The end objective of Internet communication

is a process communicating with another process. For example, computer A can

communicate with computer C by using TELNET. At the same time, computer A communicates

with computer B by using the File Transfer Protocol (FTP). For these processes

to receive data simultaneously, we need a method to label the different processes.

In other words, they need addresses. In the TCPIIP architecture, the label assigned to a

process is called a port address. A port address in TCPIIP is 16 bits in length.

Example 2.4

Figure 2.21 shows two computers communicating via the Internet. The sending computer is running

three processes at this time with port addresses a, b, and c. The receiving computer is running

two processes at this time with port addresses j and k. Process a in the sending computer needs to

communicate with process j in the receiving computer. Note that although both computers are

using the same application, FTP, for example, the port addresses are different because one is a client

program and the other is a server program, as we will see in Chapter 23. To show that data from

process a need to be delivered to process j, and not k, the transport layer encapsulates data from

the application layer in a packet and adds two port addresses (a and j), source and destination. The

packet from the transport layer is then encapsulated in another packet at the network layer with

logical source and destination addresses (A and P). Finally, this packet is encapsulated in a frame

with the physical source and destination addresses of the next hop. We have not shown the physical

addresses because they change from hop to hop inside the cloud designated as the Internet.

Note that although physical addresses change from hop to hop, logical and port addresses remain

the same from the source to destination. There are some exceptions to this rule that we discuss

later in the book.

Example 2.5

As we will see in Chapter 23, a port address is a 16-bit address represented by one decimal numher

as shown.

753

A 16-bit port address represented as one single number

Specific Addresses

Some applications have user-friendly addresses that are designed for that specific address.

Examples include the e-mail address (for example, forouzan@fhda.edu) and the Universal

Resource Locator (URL) (for example, www.mhhe.com). The first defines the recipient of

an e-mail (see Chapter 26); the second is used to find a document on the World Wide Web

(see Chapter 27). These addresses, however, get changed to the corresponding port and

logical addresses by the sending computer, as we will see in Chapter 25.