Therefore, a company which has branches in many parts of the world, can use firewalls using encryption to communicate privately and security via the Internet. This is also known as a Virtual Private Network or VPN and is substantially cheaper than setting up your own private networks using, for example, leased lines. Often a VPN is sold separately from the firewall as with eTrust. Firewalls are also able to perform user authentication, in conjunction with smart cards, tokens, biometrics etc. Further, they can be set up to protect other external connections, such as remote dial-in to the network, providing added security for the organisation.
Firewalls use one or more mechanisms to enforce policy and provide protection.
Filtering can be used to block connections to or from specific hosts, networks and ports. For example, you might wish to block connections from addresses which you consider to be untrustworthy or hostile. Or you may wish to allow SMTP traffic through one gateway and FTP through another, while blocking all other incoming traffic. Application gateways are also known as proxy servers and they are applications which filter and forward connections for services so that traffic does not get direct access to the internal hosts. The advantages of this are that the addresses of the internal systems remain hidden from external view and authentication and logging can take place at the gateway.