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Firewalls protect our computers from internet hackers. Internet hackers can steal our bank details from our computers and can reduce our bank balance from 1000s of dollars to 0 in seconds. So, firewall is must in a computer / computer network.
Starting with packet filtering firewall.
Suppose, I am downloading a file from internet. When the data-packet arrives a packet-filtering firewall, it only checks sender’s and receiver’s IP address and the port number.
If the everything is OK, then data packet is allowed to pass through packet-filtering firewall and then to my computer.
This checking process is done as per rules written in a list called access control list.
The only limitation (caveats) of packet filtering firewall is that it do not checks the data portion, i.e. Payload, of the data packet. So, a hacker could send some malicious data packed in this payload section.
The 2nd one is application / proxy firewall:
proxy firewall do not let the web server know which computer actually wants to visit the requested website, i.e. proxy firewall hides us from the attackers in internet.
Since, the application firewalls also check the data contents of the received data packet so they are generally much slower than packet-filtering firewalls.
3rd is Hybrid firewalls:
Hybrid firewalls combines packet filtering firewall and application firewall in series to enhance the security.
If they are connected in parallel then the security of the connection will be reduced to the parameters defined by the packet filtering firewall, i.e. application firewall will be of no use if the two firewalls are connected in parallel.
For this reason, hybrid firewalls use packet filtering and application firewall in series.
Please watch: "Computer Network Administrator – 5 Important Questions and answers"