Firewall

• Firewall is a security barrier between two networks that screens traffic coming in and out of the gate of one network to accept or reject connections and services according to a set of rules.
• A firewall is like a secretary for a network which examines requests for access to the network. It decides whether they pass a reasonableness test. If they pass it they are allowed through and if not they are refused.
• If a man wants to meet the chair of the community department, the secretary does a certain level of filtering but if the man wants to meet the President of the country, the secretary will perform a much different level of filtering.
• A network firewall is placed between the internal network, which might be considered safe and the external network or the Internet which is known to be unsafe.
• The job of the firewall is to determine what to let into and out of the internal network. In this way, a firewall provides access control for the network.
• There are essentially three types of firewalls. Each type of firewall filters packets by examining the data up to a particular layer of the network protocol stack.
• The firewalls are:
  • A packet filter is a firewall that operates at the network layer.
  • A stateful packet filter is a firewall that lives at the transport layer.
  • An application proxy is a firewall that operates at the application layer where it functions as a proxy.