TCP and UDP Ports Explained

  • March 24, 2004
  • Read 1,482,353 times
 

Introduction

In this tutorial we will discuss the concept of Ports and how they work with IP addresses. If you have not read our article on IP addresses and need a brush up, you can find the article here. If you understand the concepts of IP addresses, then lets move on to TCP and UDP ports and how they work.

The devices and comptuers connected to the Internet use a protocol called TCP/IP to communicate with each other. When a computer in New York wants to send a piece of data to a computer in England, it must know the destination IP address that it woud like to send the information to. That information is sent most often via two methods, UDP and TCP.

The two Internet workhorses: UDP and TCP

UDP? TCP? I know you are getting confused, but I promise I will explain this in very basic terms so that you can understand this concept.

TCP stands for Transmission Control Protocol. Using this method, the computer sending the data connects directly to the computer it is sending the data it to, and stays connected for the duration of the transfer. With this method, the two computers can guarantee that the data has arrived safely and correctly, and then they disconnect the connection. This method of transferring data tends to be quicker and more reliable, but puts a higher load on the computer as it has to monitor the connection and the data going across it. A real life comparison to this method would be to pick up the phone and call a friend. You have a conversation and when it is over, you both hang up, releasing the connection.

UDP stands for User Datagram Protocol. Using this method, the computer sending the data packages the information into a nice little package and releases it into the network with the hopes that it will get to the right place. What this means is that UDP does not connect directly to the receiving computer like TCP does, but rather sends the data out and relies on the devices in between the sending computer and the receiving computer to get the data where it is supposed to go properly. This method of transmission does not provide any guarantee that the data you send will ever reach its destination. On the other hand, this method of transmission has a very low overhead and is therefore very popular to use for services that are not that important to work on the first try. A comparison you can use for this method is the plain old US Postal Service. You place your mail in the mailbox and hope the Postal Service will get it to the proper location. Most of the time they do, but sometimes it gets lost along the way.

Now that you understand what TCP and UDP are, we can start discussing TCP and UDP ports in detail. Lets move on to the next section where we can describe the concept of ports better.

TCP and UDP Ports

As you know every computer or device on the Internet must have a unique number assigned to it called the IP address. This IP address is used to recognize your particular computer out of the millions of other computers connected to the Internet. When information is sent over the Internet to your computer how does your computer accept that information? It accepts that information by using TCP or UDP ports.

An easy way to understand ports is to imagine your IP address is a cable box and the ports are the different channels on that cable box. The cable company knows how to send cable to your cable box based upon a unique serial number associated with that box (IP Address), and then you receive the individual shows on different channels (Ports).

Ports work the same way. You have an IP address, and then many ports on that IP address. When I say many, I mean many. You can have a total of 65,535 TCP Ports and another 65,535 UDP ports. When a program on your computer sends or receives data over the Internet it sends that data to an ip address and a specific port on the remote computer, and receives the data on a usually random port on its own computer. If it uses the TCP protocol to send and receive the data then it will connect and bind itself to a TCP port. If it uses the UDP protocol to send and receive data, it will use a UDP port. Figure 1, below, is a represenation of an IP address split into its many TCP and UDP ports. Note that once an application binds itself to a particular port, that port can not be used by any other application. It is first come, first served.

 

<-------------------- 192.168.1.10 -------------------->

0 1 2 3 4 5 .. .. .. .. .. .. .. .. 65531 65532 65533 65534 65535
Figure 1. IP address with Ports

This all probably still feels confusing to you, and there is nothing wrong with that, as this is a complicated concept to grasp. Therefore, I will give you an example of how this works in real life so you can have a better understanding. We will use web servers in our example as you all know that a web server is a computer running an application that allows other computers to connect to it and retrieve the web pages stored there.

In order for a web server to accept connections from remote computers, such as yourself, it must bind the web server application to a local port. It will then use this port to listen for and accept connections from remote computers. Web servers typically bind to the TCP port 80, which is what the http protocol uses by default, and then will wait and listen for connections from remote devices. Once a device is connected, it will send the requested web pages to the remote device, and when done disconnect the connection.

On the other hand, if you are the remote user connecting to a web server it would work in reverse. Your web browser would pick a random TCP port from a certain range of port numbers, and attempt to connect to port 80 on the IP address of the web server. When the connection is established, the web browser will send the request for a particular web page and receive it from the web server. Then both computers will disconnect the connection.

Now, what if you wanted to run an FTP server, which is a server that allows you to transfer and receive files from remote computers, on the same web server. FTP servers use TCP ports 20 and 21 to send and receive information, so you won't have any conflicts with the web server running on TCP port 80. Therefore, the FTP server application when it starts will bind itself to TCP ports 20 and 21, and wait for connections in order to send and receive data.

Most major applications have a specific port that they listen on and they register this information with an organization called IANA. You can see a list of applications and the ports they use at the IANA Registry. With developers registering the ports their applications use with IANA, the chances of two programs attempting to use the same port, and therefore causing a conflict, will be diminished.

--
Lawrence Abrams
Bleeping Computer Advanced Internet Concepts Tutorial
BleepingComputer.com: Computer Support & Tutorials for the beginning computer user.

Users who read this also read:

  • IP Addresses Explained Image
    IP Addresses Explained

    Every machine on the the Internet has a unique number assigned to it, called an IP address. Without a unique IP address on your machine, you will not be able to communicate with other devices, users, and computers on the Internet. You can look at your IP address as if it were a telephone number, each one being unique and used to identify a way to reach you and only you.

  • Tracing a hacker Image
    Tracing a hacker

    Have you ever been connected to your computer when something strange happens? A CD drive opens on its own, your mouse moves by itself, programs close without any errors, or your printer starts printing out of nowhere? When this happens, one of the first thoughts that may pop into your head is that someone has hacked your computer and is playing around with you. Then you start feeling anger tinged ...

  • Understanding and Using Firewalls Image
    Understanding and Using Firewalls

    The Internet is a scary place. Criminals on the Internet have the ability to hide behind their computers, or even other peoples computers, while they attempt to break into your computer to steal personal information or to use it for their own purposes. To make matters worse, there always seems to be a security hole in your software or operating system that is not fixed fast enough that could ...

  • All about Networks Image
    All about Networks

    With so much of Computer use these days revolving around the Internet and communicating with others, its important that you understand what exactly a network is. Without networks, all communication between your computer and other computers whether it be instant messaging, email, web browsing, or downloading music could not be achieved. This tutorial will strive to teach you about networks and ...

  • The Hosts File and what it can do for you Image
    The Hosts File and what it can do for you

    When using the Internet most people connect to web sites, ftp servers or other Internet servers by connecting to a domain name, as in www.bleepingcomputer.com. Internet applications, though, do not communicate via domain names, but rather using IP addresses, such as 192.168.1.1. Therefore when you type a domain name in your program that you wish to connect to, your application must first convert ...

 

Comments:

blog comments powered by Disqus
search tutorials
Mandiant mWise Conference 2024

Login