There are a total of 65535 ports available, of which the first 1024 ports (Port 0 to 1023) are referred to as "well known" ports.
When making a connection to a particular kind of service (web, email, ftp, database servers) the software you are using will use the most common port that has been designated for that service.
Here is a list of the most commonly user ports....
- 20/21 - File Transfer Protocol (FTP): used for uploading and downloading files to and from a server.
- 25 - Simple Mail Transfer Protocol (SMTP): used for sending emails through a server.
- 53 - Domain Name System (DNS): Used by DNS servers and people requesting DNS information for a domain
- 80 - Hyper-Text Transfer Protocol (HTTP): used for serving Hyper-Text Markup Language (HTML) web pages. Even sites written in programming or scripting languages such as asp, php, or asp.net still send only HTML information to a browser
- 110 - Post Office Protocol (POP): Used for collecting emails from a server.
- 137, 138, 139 - Network Basic Input/Output System (NetBIOS): NetBIOS allows applications on separate computers to communicate over a local area network.
- 143 - Internet Message Access Protocol (IMAP): An alternative method used for collecting emails from a server, similar to the POP protocol.
- 161 - Simple Network Management Protocol (SNMP): Mostly used for remote network monitoring.
- 443 - Secure Socket Layer (SSL): Used for securing and encrypting the connection between a visitor and the server to protect the data from being read if it is intercepted.
- 587 - Alternative SMTP Port: Used as an alternative to port 25 when an ISP blocks connections on port 25.
- 1433, 1434 - SQL Server: Used by SQL server for listening for connections to a database and controlling the connection.
- 3306 - MySQL Database server: Used by MySQL for connecting to MySQL databases
- 3389 - Remote Desktop Protocol (RDP): Used to connect remotely to a server or computer via Windows Remote Desktop.
Not all of these connections are open at our firewall for all servers. We only open the minimum ports required for connection the service(s) that a server operates to reduce the attack-surface and protect our servers.