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  A firewall protects your local area network (LAN) — or even a single computer — against outside intrusion. Firewalls work as filters between the outside world and private networks, approving some types of traffic (such as when someone accesses a Web page) and blocking others (when unauthorized users attempt to access your network).
The term "firewall" doesn't actually refer to any one device. A firewall could be either a piece of hardware or software, and more than one type of firewall can be used to provide extra network security.

FireWall

 
There are two common types of firewalls:

Packet-filtering firewalls.
These firewalls apply predefined rules to filter the chunks of data, or packets, that pass through it. The filter accepts or rejects packets based on the originating computer's network address or other characteristics. The packet filter might be a computer or part of a separate piece of hardware, such as a router.

Proxy servers
Also known as gateways, a proxy server acts as a middleman that relays data between a network and the outside world. The proxy prevents outsiders from gathering information about computers inside a network. The proxy can also screen packets based on their application type (Web access or email, for example) or other identifying traits.

A firewall can also screen internal traffic on a network, separating different departments or branches of a large company. In some cases, a business will configure a proxy server to block employee access to certain types of Internet content, such as streaming audio or video.
 
Security Issues When Connecting to the Internet
When you connect your private network to the Internet, you are physically connecting your network to well over 50,000 unknown networks and all of their users. While such connections open the door to many useful applications and provide great opportunities for information sharing, most private networks contain some information that should not be shared with outside users on the Internet. In addition, not all Internet users are involved in lawful activities. These two statements foreshadow the key questions behind most security issues on the Internet:

How do you protect confidential information from those who do not explicitly need to access it?

How do you protect your network and its resources from malicious users and accidents that originate outside of your network?
 
     
 
 
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