What is a firewall and how many types of firewalls? 

Packet Filtering Firewall

Consider comparing Packet with user defined rules. If credible, packet will be sent to destination, but if not reliable, will refuse.

Circuit-level Gateway

Check the network connection path And a virtual route will be created to determine whether the incoming network is reliable or not. This type is not capable of Packet Validation itself, but Firewall Packet Validation works on a Transport Layer in the OSI Model.

what roles does a firewall perform?

 The OSI Model (Open Systems Interconnection Model) is a communication model created to communicate with each other between networks.There are seven functional layers, each with different functions, while the Transport Layer is one of the OSI models. Checked page And fix any mistakes that occur in packet delivery. Identify and organize packets in correct order.

Stateful Inspection Firewall

This type of monitoring not only checks the packet, but also tracks whether the packet has ever entered this network. Or having entered for the first time It will take into account the packet information and the information obtained from the previous packets. This type is more secure than route validation or packet filtering alone.

Application-level Gateway

It is a type of firewall that is installed on a computer, separated from the router, but also connected to the router to find the path of transmission. Packet acts as a filter. And monitoring the contents of the Packet can detect The OSI Model sometimes acts like a Proxy Firewall, which is a network security system that protects network data by controlling it. And check any abnormal data