How Firewalls Work

How Firewalls WorkIf you have been using Internet on a regular basis or working in a large company and surf the Internet while you are at work, you must have surely come across the term firewall.

You might have also heard of people saying “firewalls protect their computer from web attacks and hackers” or “a certain website has been blocked by firewall in their work place”. If you have ever wondered to know what exactly is this firewall and how it works, here we go. In this post I will try to explain “How firewalls work” in layman’s terms.

How Firewalls Work?

Firewalls are basically a barrier between your computer (or a network) and the Internet (outside world). A firewall can be simply compared to a security guard who stands at the entrance of your house and filters the visitors coming to your place. He may allow some visitors to enter while denying others whom he suspects of being intruders. Similarly a firewall is a software program or a hardware device that filters the information (packets) coming through the Internet to your personal computer or a computer network.

The working of a Firewall

Firewalls may decide to allow or block network traffic between devices based on the rules that are pre-configured or set by the firewall administrator. Most personal firewalls such as Windows firewall operate on a set of pre-configured rules that are most suitable under normal circumstances so that, the user need not worry much about configuring the firewall.

Personal firewalls are easy to install and use and hence preferred by end-users for use on their personal computers.  However, large networks and companies prefer those firewalls that have plenty of options to configure so as to meet their customized needs.

For example, a company may set up different firewall rules for FTP servers, Telnet servers and Web servers. In addition, the company can even control how the employees connect to the Internet by blocking access to certain websites or restricting the transfer of files to other networks. Thus, in addition to security, a firewall can give the company a tremendous control over how people use the network.

Firewalls use one or more of the following methods to control the incoming and outgoing traffic in a network:

  1. Packet Filtering: In this method, packets (small chunks of data) are analyzed against a set of filters. Packet filters has a set of rules that come with accept and deny actions which are pre-configured or can be configured manually by the firewall administrator. If the packet manages to make it through these filters then it is allowed to reach the destination; otherwise it is discarded.

  2. Stateful Inspection: This is a newer method that doesn’t analyze the contents of the packets. Instead, it compares certain key aspects of each packet to a database of trusted source. Both incoming and outgoing packets are compared against this database and if the comparison yields a reasonable match, then the packets are allowed to travel further. Otherwise they are discarded.

Firewall Configuration:

Firewalls can be configured by adding one or more filters based on several conditions as mentioned below:

    1. IP addresses: In any case, if an IP address outside the network is said to be unfavorable, then it is possible to set  filter to block all the traffic to and from that IP address. For example, if a certain IP address is found to be making too many connections to a server, the administrator may decide to block traffic from this IP using the firewall.

    2. Domain names: Since it is difficult to remember the IP addresses, it is an easier and smarter way to configure the firewalls by adding filters based on domain names. By setting up a domain filter, a company may decide to block all access to certain domain names, or may provide access only to a list of selected domain names.

    3. Ports/Protocols: Every service running on a server is made available to the Internet using numbered ports, one for each service. In simple words, ports can be compared to virtual doors of the server through which services are made available.

      For example, if a server is running a Web (HTTP) service then it will be typically available on port 80. In order to avail this service, the client needs to connect to the server via port 80. Similarly, different services such as Telnet (Port 23), FTP (port 21) and SMTP (port 25) services may be running on the server.

      If the services are intended for the public, they are usually kept open. Otherwise they are blocked using the firewall so as to prevent intruders from using the open ports for making unauthorized connections.

    4. Specific words or phrases: A firewall can be configured to filter one or more specific words or phrases so that, both the incoming and outgoing packets are scanned for the words in the filter.

      For example, you may set up a firewall rule to filter any packet that contains an offensive term or a phrase that you may decide to block from entering or leaving your network.

Hardware vs. Software Firewall:

Hardware firewalls provide higher level of security and hence preferred for servers where security has the top most priority. The software firewalls on the other hand are less expensive and hence preferred in home computers and laptops.

Hardware firewalls usually come as an in-built unit of a router and provide maximum security as it filters each packet in the hardware level itself even before it manages to enter your computer. A good example is the Linksys Cable/DSL router.

Why Firewall?

Firewalls provide security over a number of online threats such as Remote login, Trojan backdoors, Session hijacking, DOS & DDOS attacks, viruses, cookie stealing and many more. The effectiveness of the security depends on the way you configure the firewall and how you set up the filter rules.

However, major threats such as DOS and DDOS attacks may sometimes manage to bypass the firewalls and do the damage to the server. Even though firewall is not a complete answer to online threats, it can most effectively handle the attacks and provide security to the computer up to the maximum possible extent.


  1. Pratik waikar
    November 19, 2010 at 10:49 PM

    Thanks a lot for sharing such useful information….can u tell me how to disable firewall in collage ?? The collage have blocked sme websites n i want to açess that websites , so can u tell me how to disable firewall ?? Thanks…

  2. mrlmohan
    November 20, 2010 at 9:18 AM

    nice post…

    Which is best free firewall available to download from internet?…

    • Srikanth
      November 22, 2010 at 11:43 AM

      @ mrlmohan

      Zone Alarm is the best firewall out there on the Internet.

  3. fred
    November 21, 2010 at 1:02 AM

    u re doing a great job………. i knew alot studying this webpages.
    am truly growing… thanks for your benevolence.. may GOD reward you and your purse…….

  4. Mars M
    November 21, 2010 at 7:35 PM


  5. Deep
    December 9, 2010 at 3:29 PM

    Thank you sir……………

  6. akshay
    December 30, 2010 at 4:45 PM

    thanks for the info it is very interesting

  7. seji
    January 9, 2011 at 8:39 PM

    Thank you sir.I can improve my knowledge by reading this article.

  8. hardik
    January 10, 2011 at 3:12 PM

    thanks for providing useful information… its very helpful for me to understand wat the firewall is??..

  9. winko
    January 18, 2011 at 9:04 AM

    Thank you. This site is very good.

  10. Meenja
    January 27, 2011 at 11:59 AM

    Thank you for the information, it’s nice and informative, thanks a lot again.

  11. Sumeet Kukreja
    April 23, 2011 at 11:46 AM

    Zone alarm is the best firewall available at the internet? i agree but it has many problems… For example… it is not opening in “Win 7 x64″ and It slowdown startup of XP SP3 (On XP sp2 it works fine).
    Instead of Zone Alarm i think Comodo firewall is a good choice

  12. Dr. Gene Ray
    April 28, 2011 at 11:41 AM

    Great article, fast shipping, great to deal with! A+++++++++++ WILL READ AGAIN

  13. Sudeep Acharya
    May 29, 2011 at 8:26 AM

    I am using avast should i need to use firewall programs with avast

    • Srikanth
      May 30, 2011 at 10:39 AM

      @ Sudeep Acharya

      It is better to install a seperate firewall unless it is not bundled with the antivirus program itself. Anyway Windows has a built in firewall which gives adequate protection.

  14. siva_phd9
    June 1, 2011 at 9:35 PM


  15. techie
    July 13, 2011 at 11:09 AM

    Great info.Thanks .Mnay webistes were visited , bt none explained better than this .

  16. Bhawaria
    July 23, 2011 at 1:58 PM

    Thanks Bro,,,,,,,,,,,,,,

  17. Abhishek
    August 22, 2011 at 4:36 PM

    Thanks Bro,

    You can also check out for IPcop standalone firewall.

  18. Sampath
    August 27, 2011 at 11:57 AM

    Nice job dude.can u tell me some information about proxy server

  19. zero cool
    September 9, 2011 at 2:49 PM

    online armour is the best free comes zonealarm

  20. Billy Tyson
    October 20, 2011 at 1:23 PM

    nice post yar.. i am using k7 antivirus it has a built in firewall..

  21. vishnu
    October 17, 2012 at 10:00 AM

    one person in yahoo told me that he will hack my ip adders and
    contact..through my past yahoo messages to him is it possible..what should i install to my pc …to over come these thing..please help..:(…please send reply to my e-mail if not here..

  22. KIRAN
    November 24, 2012 at 8:14 AM

    Dude can u tel me How to open websites if blocked by firewall without leaving any history in company and college….

  23. Lonelywolf
    November 29, 2012 at 7:01 AM

    Srikanth is there anyway by passing a firewall either software\Hardware. At least be able to Skip a specific file from been filtered.

  24. Bhaskar
    May 5, 2013 at 10:34 AM

    Thanks. It is useful information

  25. syed meraj
    November 1, 2014 at 9:24 AM

    its awsome about this thing its greatful information to boost knowledge

Leave A Reply