Firewall Systems

A firewall is part of a network that blocks unauthorized access while permitting authorized access to the network. The firewall does this by blocking ports which are associated with applications or by blocking IP addresses. Firewalls are either hardware or software or a combination. Firewalls are often used to prevent unauthorized users on the Internet from gaining access to an intranet whether that be a LAN or WAN. They are also used to block users inside the network from accessing specified content on the Internet. All information inbound to intranet (your organization's network) or outbound to the Internet passes through the firewall, which examines every message, preventing those messages that do not fit the criteria from being sent or received and forwarding messages that do fit the criteria onward.

There are several types of methods firewalls use:

  • Packet filtering
    Inspects each packet and accepts or rejects based on defined rules. Complex to configure, but fairly effective and transparent to users, it can be vulnerable to IP spoofing however.
  • Application gateway
    This applies security mechanisms to specific applications, like Telnet and FTP servers. This is very effective but can slow data traffic.
  • Circuit-level gateway
    Applies security mechanisms when a TCP or UDP connection is established. Once the connection has been made, data packets can flow between the hosts without further checking.
  • Proxy server
    Intercepts all messages entering and leaving the network. The proxy server essentially hides the true network addresses and adds another layer of protection.


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