• 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

• Figure 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.
•  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.
• a port address is a 16-bit address represented by one decimal number as shown.753

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
• the second is used to find a document on the World Wide Web
• These addresses, however, get changed to the corresponding port and logical addresses by the sending computer.

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