Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa. Stateful Inspection - A newer method that doesn't examine the contents of each packet but instead compares certain key parts of the packet to a database of trusted information. Information traveling from inside the firewall to the outside is monitored for specific defining characteristics, and then incoming information is compared to these characteristics. If the comparison yields a reasonable match, the information is allowed through. Otherwise it is discarded.
Firewalls are customizable.
This means that you can add or remove filters based on several conditions. For example, if a certain IP address outside the company is reading too many files from a server, the firewall can block all traffic to or from that IP address. Because it is hard to remember the string of numbers that make up an IP address, and because IP addresses sometimes need to change, all servers on the Internet also have human-readable names, called domain names. A company might block all access to certain domain names, or allow access only to specific domain names. Protocols are often text, and simply describe how the client and server will have their conversation.