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Port Blocking

Port Blocking is the restriction of internet services on certain ports. A port is a data connection that can be used by programs to exchange data directly, instead of going through a file or other temporary storage location.
After you install Windows XP Service Pack 2 (SP2), some programs may seem not to work. By default, Windows Firewall is enabled and blocks unsolicited connections to your computer. This article discusses how to make an exception and enable a program to run by adding it to the list of exceptions. This procedure enables the program to work as it did before the service pack was installed.

To help provide security for your Windows XP SP2-based computer, Windows Firewall blocks unsolicited connections to your computer. However, sometimes you might want to make an exception and allow for someone to connect to your computer. For example, the following scenarios describe occasions when you might want someone to be able to connect to your computer:
  •    You are playing a multilayer game over the Internet.
  •    You are expecting to receive a file that is sent through an instant message program.

After you install Windows XP SP2, client applications may not successfully receive data from a server. The following are some examples:
  • An FTP client
  • Multimedia streaming software
  • New mail notifications in some e-mail programs

Or, server applications that are running on a Windows XP SP2-based computer may not respond to client requests. The following are some examples:

  •   A Web server such as Internet Information Services (IIS)
  •   Remote Desktop
  •   File Sharing


Article source: http://www.sky.com/helpcentre/glossary/index.html
Article source: http://support.microsoft.com/kb/842242