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Ports & Services

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This is not intended to be an exhaustive list of all ports and Application level protocols, just the more common ones you will encounter.

What are ports?

There are 65355 virtual software ports, they are split up into three groups: the Well Known ports (1-1023), the Registered ports (1024 - 49151), and the Dynamic and/or Private ports (49152-65535). They don't physically exist like USB or Printer ports that you will find on the back of your computer. They are also referred to as sockets and are end points on your computer for TCP/IP and UDP connections. The port number identifies what type of port it is, common ones your most likely to incounter are listed below.

Common Ports and Services you are likely to encounter:

FTP
FTP DATA
HTTP
HTTPS
IMAP
NNTP
POP3
PROXY
SMTP

FTP (File Transfer Protocol)

Asscociated port: 21

This is the service used for FTP control connections. Transferring of files can be done via HTTP, but FTP is usually used and can be done in a dedicated FTP Client (like LeechFTP or Cute FTP) or a browser.

FTP uses two channels to achieve the transfer, there is the control connection (using the FTP service) which is used to send the necessary commands to achieve the transfer and a data channel (see FTP DATA) which is used to actually send the files with. The control connection is established from the client when the client logs into an ftp server, whereas the data channel is usually established by the server to the client.
Ports & Services

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FTP DATA (File Transfer Protocol Data)

Associated port: 20

This is the service used for FTP data connections. Transferring of files can be done via HTTP, but FTP is usually used and can be done in a dedicated FTP Client (like LeechFTP or Cute FTP) or a browser.
For more information see FTP.
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HTTP (Hyper Text Transfer Protocol)

Associated port: 80

Your browser is using this service now to show you this page. HTTP is the service used by all browsers or browser-like applications to retrieve web pages and other files, other applications may also use HTTP such as e-mail clients so that they can collect all the elements required to display HTML formatted e-mail. HTTP defines how messages and commands are handled and what actions web servers and clients (like the browser your viewing this page with now) should take in response. When a client, like your browser, sends a request to an web server it builds and HTTP request and sends this to the IP address indicated by the URL. An HTTP daemon running on the server queues the requests and after performing any necessary processing, returns the requested file. HTTP is stateless, this means that the commands sent via HTTP by a client to a server are independent of each other, they have no knowledge of the commands sent before it. HTTP is therefore not very good at reacting to user input, to counter this several other technologies have been created, these include, Cookies, Java, JavaScript, and ActiveX.
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HTTPS (Hypertext Transfer Protocol over Secure Socket Layer (SSL))

Associated port: 443

This service is used by browsers and browser-like applications when the transmitted information needs to be protected. You can tell when you are using a secure connection because an icon in your browser representing a padlock appears closed as opposed to open (when using HTTP). SSL has been succeeded by TLS (Transport Layer Security), but the port and service name are the same. It should be noted that HTTPS is not the same as S-HTTP. HTTPS is commonly used when using banking or shopping services or when a password is required, for example, when you log in to Hotmail.
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IMAP (Internet Message Access Protocol)

Associated port: 143

IMAP, like POP3 is a service used for receiving e-mail, but differs in that there are some useful additional features like the ability to search through your mail while it is still on your mail server and delete unwanted mail. IMAP is used to manipulate your mail while it is still on the remote server, allowing you to create folders, delete mail etc. without downloading it. Unlike POP3, if your e-mail client uses IMAP it downloads just the headers to show you the subject and sender etc., this is the opposite of POP3 which downloads the header of an e-mail and the body along with attachments. You then choose from the header information which mails you want to view and your e-mail client then downloads the message.
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NNTP (Network News Transfer Protocol)

Associated port: 119

NNTP is the protocol used by your e-mail client or newsreader to read newsgroup posts. The protocol collects headers of all new messages on the news server and displays them in your news reader. YOu then select a post and the post's body section is downloaded so that you may read them.
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POP3 (Post Office Protocol - version 3)

Associated port: 110

POP3 is a service used to collect mail from a mail server. An earlier version of POP (POP2) became popular in the mid-eighties required the use of SMTP to send mail. POP3 can be used with or without SMTP. POP3 is now the standard service used to collect mail by e-mail clients today and the service can be thought of as a 'store and forward service', the mail is sent to your pop server where it is stored until you request it through an e-mail client. It is then forwarded to your e-mail client.
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PROXY

Associated ports: 3128, 8008, 8088, and 8080

The proxy ports are the commonly used ports that proxy servers listen to for connections. The above listed ports are the most commonly used although most proxy servers allow the port they listen on for connections to be defined by the user. Proxy servers are used on networks and allow access to the Internet to computers on the network that do not have a connection to the Internet by routing the connection through a computer on the network that does have a connection to the Internet If for example, you had two or more computers but only one Internet connection you would normally be only able to access the Internet from one of those computers. However if you created a small LAN (Local Area Network) with your computers you could run a proxy server (like this excellent free one form AnalogX) on the computer with the Internet connection and all of your computers would then have Internet access.
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SMTP (Simple Mail Transfer Protocol)

Associated port: 25

SMTP is used to send mail between servers, it is also the service used when sending mail from your e-mail client. It can also be used to retrieve mail but because it is has limited ability to queue mail at the receiving end it is often used along side either POP3 or IMAP to collect the mail.
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This is an unofficial guide, the information expressed here may differ from Agnitum's. There is a support forum (no longer run by Agnitum, but by users) if you need more help this is a good place to start. Where information here conflicts with what Agnitum have told you always go with the information given to you by Agnitum.

Guide/site and images ©Stephen Cox