Please Whitelist This Site?

I know everyone hates ads. But please understand that I am providing premium content for free that takes hundreds of hours of time to research and write. I don't want to go to a pay-only model like some sites, but when more and more people block ads, I end up working for free. And I have a family to support, just like you. :)

If you like The TCP/IP Guide, please consider the download version. It's priced very economically and you can read all of it in a convenient format without ads.

If you want to use this site for free, I'd be grateful if you could add the site to the whitelist for Adblock. To do so, just open the Adblock menu and select "Disable on tcpipguide.com". Or go to the Tools menu and select "Adblock Plus Preferences...". Then click "Add Filter..." at the bottom, and add this string: "@@||tcpipguide.com^$document". Then just click OK.

Thanks for your understanding!

Sincerely, Charles Kozierok
Author and Publisher, The TCP/IP Guide


NOTE: Using software to mass-download the site degrades the server and is prohibited.
If you want to read The TCP/IP Guide offline, please consider licensing it. Thank you.

The Book is Here... and Now On Sale!

The whole site in one document for easy reference!
The TCP/IP Guide

Custom Search







Table Of Contents  The TCP/IP Guide
 9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)
      9  TCP/IP Transport Layer Protocols
           9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)
                9  TCP/IP Transport Layer Protocol (TCP and UDP) Addressing: Ports and Sockets

Previous Topic/Section
TCP/IP Processes, Multiplexing and Client/Server Application Roles
Previous Page
Pages in Current Topic/Section
1
23
Next Page
TCP/IP Application Assignments and Server Port Number Ranges: Well-Known, Registered and Dynamic/Private Ports
Next Topic/Section

TCP/IP Ports: Transport Layer (TCP/UDP) Addressing
(Page 1 of 3)

A typical host on a TCP/IP internetwork has many different software application processes running concurrently. Each generates data that it sends to either TCP or UDP, which in turn passes it to IP for transmission. This multiplexed stream of datagrams is sent out by the IP layer to various destinations. Simultaneously, each device's IP layer is receiving datagrams that originated in numerous application processes on other hosts. These need to be demultiplexed, so they end up at the correct process on the device that receives them.

Multiplexing and Demultiplexing Using Ports

The question is: how do we demultiplex a sequence of IP datagrams that need to go to many different application processes? Let's consider a particular host with a single network interface bearing the IP address 24.156.79.20. Normally, every datagram received by the IP layer will have this value in the IP Destination Address field. Consecutive datagrams received by IP may contains a piece of a file you are downloading with your Web browser, an e-mail sent to you by your brother, and a line of text a buddy wrote in an IRC chat channel. How does the IP layer know which datagrams go where, if they all have the same IP address?

The first part of the answer lies in the Protocol field included in the header of each IP datagram. This field carries a code that identifies the protocol that sent the data in the datagram to IP. Since most end-user applications use TCP or UDP at the transport layer, the Protocol field in a received datagram tells IP to pass data to either TCP or UDP as appropriate.

Of course, this just defers the problem to the transport layer: both TCP and UDP are used by many applications at once. This means TCP or UDP must figure out which process to send the data to. To make this possible, an additional addressing element is necessary. This address allows a more specific location—a software process—to be identified within a particular IP address. In TCP/IP, this transport layer address is called a port.

Key Concept: TCP/IP transport layer addressing is accomplished using TCP and UDP ports. Each port number within a particular IP device identifies a particular software process.



Previous Topic/Section
TCP/IP Processes, Multiplexing and Client/Server Application Roles
Previous Page
Pages in Current Topic/Section
1
23
Next Page
TCP/IP Application Assignments and Server Port Number Ranges: Well-Known, Registered and Dynamic/Private Ports
Next Topic/Section

If you find The TCP/IP Guide useful, please consider making a small Paypal donation to help the site, using one of the buttons below. You can also donate a custom amount using the far right button (not less than $1 please, or PayPal gets most/all of your money!) In lieu of a larger donation, you may wish to consider purchasing a download license of The TCP/IP Guide. Thanks for your support!
Donate $2
Donate $5
Donate $10
Donate $20
Donate $30
Donate: $



Home - Table Of Contents - Contact Us

The TCP/IP Guide (http://www.TCPIPGuide.com)
Version 3.0 - Version Date: September 20, 2005

Copyright 2001-2005 Charles M. Kozierok. All Rights Reserved.
Not responsible for any loss resulting from the use of this site.