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!

Get The TCP/IP Guide for your own computer.
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 Application Assignments and Server Port Number Ranges: Well-Known, Registered and Dynamic/Private Ports
Previous Page
Pages in Current Topic/Section
1
23
Next Page
TCP/IP Sockets and Socket Pairs: Process and Connection Identification
Next Topic/Section

TCP/IP Client (Ephemeral) Ports and Client/Server Application Port Use
(Page 1 of 3)

The significance of the asymmetry between clients and servers in TCP/IP becomes evident when we examine in detail how port numbers are used. Since clients initiate application data transfers using TCP and UDP, it is they that need to know the port number of the server process. Consequently, it is servers that are required to use universally-known port numbers. Thus, well-known and registered port numbers identify server processes. They are used as the destination port number in requests sent by clients.

In contrast, servers respond to clients; they do not initiate contact with them. Thus, the client doesn't need to use a reserved port number. In fact, this is really an understatement: a server shouldn't use a well-known or registered port number to send responses back to clients. The reason is that it is possible for a particular device to have both client and server software of the same protocol running on the same machine. If a server received an HTTP request on port 80 of its machine and sent the reply back to port 80 on the client machine, it would be sending the reply to the client machine's HTTP server process (if present) and not the client process that sent the initial request.

To know where to send the reply, the server must know the port number the client is using. This is supplied by the client as the Source Port in the request, and then used by the server as the destination port to send the reply. Client processes don't use well-known or registered ports. Instead, each client process is assigned a temporary port number for its use. This is commonly called an ephemeral port number.

Note: Your $10 word for the day: ephemeral: “short-lived; existing or continuing for a short time only.” — Webster's Revised Unabridged Dictionary.



Previous Topic/Section
TCP/IP Application Assignments and Server Port Number Ranges: Well-Known, Registered and Dynamic/Private Ports
Previous Page
Pages in Current Topic/Section
1
23
Next Page
TCP/IP Sockets and Socket Pairs: Process and Connection Identification
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.