• Packet Filter: Although this technique is difficult to configure and vulnerable to denial of service attack it is fairly effective and transparent to users. The way it works is that each packet leaving or entering a network is screened based on the rules set by the network administrator. The packet we are talking about here is a message that is transmitted over a network and contains both a destination address as well as data.
• Proxy Server: A proxy server is a server that sits between a client application, such as a Web browser, and a real server. This method intercepts all messages entering and leaving a network. The proxy server essentially hides the network's true address.
• Ports: A server machine makes its services available to the Internet using numbered ports for e.g. if a server machine is running a Web (HTTP) server and an FTP server, the Web server would typically be available on port 80, and the FTP server would be available on port 21. Depending on the requirement these ports could then be blocked. Unless you're running a public web site or FTP site on your corporate network (which you really shouldn't be doing), keeping HTTP and FTP open is just asking for trouble.
• Protocols: These simply describe how the client and server will communicate and are normally pure text based. Firewall filters can be programmed to block or allow certain protocols like TCP and UDP among others. For example, blocking ICMP means that nobody will be able to ping your IP address.
• IP addresses: This can be used in two ways. You can block IP addresses of certain websites from being accessed through a corporate network. The second way is to block the IP address of a machine that you think poses a security risk. If you found, for instance, that somebody was doing port scans from a particular IP address, it would probably be a good idea to block that IP address completely.

• Network Layer Firewall: These types of firewalls make their decision based on the source, the destination addresses and ports. Unlike routers which are in a small way like network layer firewalls minus the packet source and destination info, network layer firewalls maintain internal information about the state of connections passing through them, the contents of some of the data streams, and so on. An important distinction about many network layer firewalls is that they route traffic directly though them, so to use one you either need to have a validly assigned IP address block or to use a ``private internet'' address block such as a 10.x.x.x or a 198.x.x.x address block. Network layer firewalls tend to be very fast and are transparent to users. In such a screened host firewall, access to and from a single host is controlled by means of a router operating at a network layer. The single host is a bastion host; a highly defended and secured strong point that can resist attack.
• Application Layer Firewall: These are generally hosts running proxy servers, which permit no traffic directly between networks, and which perform elaborate logging and auditing of traffic passing through them. A proxy server is a server that sits between a client application, such as a Web browser, and a real server. Proxies are often used instead of router-based traffic controls, to prevent traffic from passing directly between networks. Many proxies contain extra logging or support for user authentication. This method intercepts all messages entering and leaving a network. The proxy server essentially hides the network's true address.

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