A firewall is a network security system, either hardware or software based that controls incoming and outgoing network traffic based on a set of rules.
Firewall technology developed in the late 1980s.
Firewalls work like a filter between your computer/network and the Internet.
You can program what you want to get out and what you want to get in. Everything else is not allowed.
Types of firewall
Packet/network firewalls
These are the earliest firewalls functioned as packet filters, inspecting the packets that are transferred between computers on the Internet.
When a packet passes through a packet filter firewall, its source and destination address, protocol and destination port number are checked against the firewall's rule set.
Stateful firewalls
These firewalls used to recognize a packet's connection state.
First introduced in 1994 by Check Point Software.
The additional information can be used to grant or reject access based on the packet's history in the state table, and to speed up packet processing.
So the packets those are part of an existing connection based on the firewall's state table can be allowed through without further analysis.
Application layer firewalls
These firewalls protect servers and the applications running on them, not the network resources behind them.
This technology first developed in 1999.
These firewalls inspect and filter packets on any OSI layer up to the application layer.
These firewalls have ability to block specific content, such as known malware or certain websites, and recognize when certain applications and protocols being misused.
Proxy firewalls
A proxy firewall is a network security system that protects network resources by filtering messages at the application layer.
Proxy firewalls are considered to be the most secure type of firewall because they prevent direct network contact with other systems.
A proxy firewall acts as an intermediary between in-house clients and servers on the Internet.
Advantages
Monitors traffic
At the time of transmission of packets through the network firewalls investigates to determine whether they something contain that's potentially hazardous to your network’s security.
Block Trojans
A firewall helps block Trojan horses.
Trojans are specially dangerous because they silently transmit what they uncover about you to a Web server.
Stop hackers
Having a firewall keeps hackers out of your network.
Without firewall security, a hacker could get a hold of your computer and make it a part.
Stop key loggers
Having firewall security will reduce the risk of key loggers monitoring you.
A key logger is spyware software that cyber criminals try to put on your computer so they can target your keystrokes.
So they can know what you are typing, and because of this information they can access your private account.
Disadvantages
Firewalls can degrade your system performance, because it's a demanding task to examine every packet of data.
Configuring and maintaining of firewalls are difficult.
Sometimes difficult to remove or uninstall a firewall completely.
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