What is firewall??

A firewall is simply a program or hardware device that filters the information
coming through the Internet connection into your private network.

Firewalls use one or more of three methods to control traffic flowing in and out of the network:

* Packet filtering -packet filtering is the process of passing or blocking packets at a network interface based on source and destination addresses, ports, or protocols.

* Proxy service - Information from the Internet is retrieved by the firewall and then sent to the requesting system and vice versa.

* Stateful inspection -Statefull firewall does deep packet inspection & forwards the traffic
They watch the traffic from end to end and keeps track of open connection on state table.
The stateful firewall depends on the famous {SYN, SYN-ACK, ACK} three-way handshake of the TCP protocol