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In computer networks, a firewall is a set of information security functions that may or may not reside in a single physical computer. Large or fault-tolerant networks have multiple firewalls. The functions control certain types of access to the protected network. Most often, we think of the attacks as coming from the Internet, but firewalls have applications for internal networks — and not all secure access through the Internet goes through a firewall.

Some basic firewall functions include:

What a firewall is not

  • A substitute for a security policy
  • A substitute for information security administration
  • (necessarily) a single computer
  • A guarantee of network security

Firewalls are not primary protection against

Cooperating services

These may or may not share computers, although it is wise to have the minimum possible number of services on a firewall.