What is the principle of firewall technology in a computer network system?

A firewall is an access control technology that is used to strengthen border defenses between two or more networks. It works as follows: a separation wall is set up between a public **** network and a private network, where it is checked whether information to and from the private network is allowed to pass through, or whether a user's request for service is allowed, thus preventing illegal access to information resources and the entry of unauthorized users. This is a passive defense technique. The establishment of a firewall requires the network to have a clear boundary and service type, so that it can isolate the internal and external networks to achieve the purpose of protection.

A firewall does not just refer to a host, router, or multi-computer system used to provide security for a network. It should be said that a firewall is a means of security that helps establish a network security protocol and implements that security protocol through network configuration, host systems, routers, and means such as authentication. The host goal of a firewall system is to control access to and from a network, and it forces all connections to pass through the firewall in order to be inspected.

A firewall system can be a router, a host, or a cluster of hosts, and they are built to prevent a subnet that needs to be protected from being interfered with, or disrupted, by factors outside the subnet. Firewalls are typically built on high-level gateways -- such as a site's connection to the Internet. However, firewalls can also be placed at the boundaries of a subnet to protect a subnet or part of a group of hosts.