The Transmission Control Protocol / Internet Protocol (TCP/IP) is the communication protocol by which packets of data are sent and received over the Internet by server and client computers. As there are a vast many different types of data contained in these packets, different ports, classified by number, are used to separate the types of services which data is sent and received between server and client computers.

A server can be set to accept or deny requests for these specific packets of data by making the port used to receive the data available on the server itself. Firewalls are used to specify which ports are available to accept incoming and outgoing connections on any particular computer.

Ports are designated by the Internet Assigned Numbers Authority, the entity responsible for the global coordination of the DNS Root, IP addressing, and other Internet protocol resources. There are a total of 65535 ports available for use in the TCP/IP framework, and ports 0-1023 are considered 'well known' ports.

Here is a list of ports which are commonly used and essential in server-side Internet communications:

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 for the translation of domain names to the IP addresses they are assigned to, employing the use of Name Servers.

80 - Hyper-Text Transfer Protocol (HTTP): used for serving Hyper-Text Markup Language (HTML) based web pages, and other types of web site files, such as Active Server Pages (ASP).

110 - Post Office Protocol (POP): Used for receiving 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): Alternate method used for receiving emails from a server, similar to the POP protocol.

161 - Simple Network Management Protocol (SNMP): Used primarily for the remote network monitoring of a server or cluster of servers, to ensure up-time and responsiveness.

443 - Secure Socket Layer (SSL): Used for securing and encrypting the connection from a user's computer to a server in order to protect the packet data being transmitted.

445 - Server Message Block (SMB): Used for Microsoft Windows Networking communication. Essential for File and Printer Sharing, and connecting to a server's shared resources via Uniform Naming Convention (UNC) path. It is also essential for network communication on servers utilizing Active Directory.

587 - Alternate SMTP: Commonly used as a viable alternative to port 25, as some Internet Service Providers choose to block connectivity on port 25.

1167 - Continuous Data Protection (CDP): Used for the R1Soft Backup Agent for processing automated off-site server backups.

2433 - Microsoft Structured Query Language Server (SQL Server): Used for database connectivity between server and client computers. The port historically used for SQL communications is 1433, however port 1433 is often the subject of brute-force attacks, thus, we at Server Intellect choose to alter the SQL port to 2433 in an effort to reduce the amount of attacks against our servers.

3389 - Remote Desktop Protocol (RDP): Used for connecting remotely to a server or client computer via Windows Remote Desktop.