What is a firewall and how does a firewall work?

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Aina Prat Blasi
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Un firewall is the first line of defense that protects your network. As you learn the essentials of computer security and the network, you will encounter many new terms: encryption, ports, Trojans and others. Firewall is a term that will pop up again and again.


What is a firewall?

A firewall is the first line of defense for your network. The main purpose of a firewall is to prevent uninvited guests from browsing your network. A firewall can be a hardware device or a software application that is usually placed on the perimeter of the network to act as a gatekeeper for all inbound and outbound traffic.



A firewall allows you to establish certain rules to identify the traffic that should be allowed inside or outside the private network. Depending on the type of firewall you implement, you can restrict access to only certain IP addresses and domain names, or you can block certain types of traffic by blocking the TCP / IP ports they use.

How does a firewall work?

There are basically four mechanisms used by firewalls to restrict traffic. A device or application can use more than one of these to provide in-depth protection. The four mechanisms are packet filtering, network level gateway, proxy server, and gateway.


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Packet filter

A packet filter intercepts all traffic to and from the network and evaluates it according to the rules provided. Typically, the packet filter can evaluate the source IP address, source port, destination IP address, and destination port. These are the criteria that can be filtered to allow or prevent traffic from certain IP addresses or on certain ports.



Circuit-level gateway

A circuit-level gateway blocks all inbound traffic to any host but itself. Internally, client machines run software to allow them to establish a circuit-level connection with the gateway machine. To the outside world, it appears that all communication from the internal network is coming from the gateway at the circuit level.

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proxy server

A proxy server is usually created to improve network performance, but it can also act as a kind of firewall. Proxy servers hide your internal addresses so that all communication appears to come from the proxy server itself. A proxy server caches the pages that have been requested.


If user A goes to Yahoo.com, the proxy server sends the request to Yahoo.com and retrieves the web page. If user B then connects to Yahoo.com, the proxy server simply sends the information it already has. retrieved for user A, so it is returned much faster than having to get it back from Yahoo.com.

You can configure a proxy server to block access to certain websites and filter traffic from certain ports to protect the internal network.

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Application gateway

An application gateway is essentially another type of proxy server. The internal client first establishes a connection with the Application Gateway. Application Gateway determines whether the connection should be allowed or not and then establishes a connection with the target computer.



All communication goes through two connections: client to application gateway and application gateway to destination. Application Gateway monitors all traffic against its rules before deciding whether to forward it. As with other types of proxy servers, Application Gateway is the only address seen by the outside world, so the internal network is protected.


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Further Reading:

  • How to disable Microsoft Defender Firewall in Windows 11
  • How to enable the firewall built into your wireless modem router
  • How to disable Windows Firewall in Windows 10, 8, 7, Vista and XP
  • 2 methods to enable or disable Windows 10 firewall notifications
  • 4 Best Firewalls for Android Phone

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