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The most effective way to secure an Internet link is to put a firewall between the local network and the Internet. A firewall is a system designed to prevent unauthorized access to or from a secured network. Firewalls act as locked doors between internal and external networks: data that meets certain requirements is allowed through, while unauthorized data is not.
To provide robust security, a firewall must track and control the flow of communication passing through it. To reach control decisions for TCP/IP-based services, (such as whether to accept, reject, authenticate, encrypt, and/or log communication attempts), a firewall must obtain, store, retrieve, and manipulate information derived from all communication layers and other applications.