Malicious users or attackers on the Internet create worms and viruses that can reveal or destroy valuable data, and they run tools that attempt to break into your computer. To do this, the tools, viruses, and worms send messages to your client computer, addressed to the ports that various programs use to receive legitimate messages. If the malicious code is able to make contact at a particular port, it may be able gain entrance to your computer. To limit this security threat, you can enable firewall software, which blocks all ports except the ones you intentionally open. Internet Connection Firewall (ICF) is firewall software that is supplied with Microsoft Windows XP.

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