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A firewall is a hunk of a computer system and network that is especially designed to block unlicensed access in to the computer, especially while using internet service. It is a gadget or set of software programming, where it blocks unwanted software or tools accessing into the computer.
The notion of the firewall which comes from the erstwhile antiquities. These believes are not relates to the research background, but the coincidence and imagination points were taken accounts. For example, in excess of two thousand eons ago, the Chinese built the Great Wall as protection from bordering northerly people to rescue China. Another example is that of European kings who built forts with high walls and dikes to protect themselves and their issues, both from attacking armies and from robbing bands intent on stealing and looting. Firewall diagram which displayed below,
 Retrieved on 25 Dec'10 by, http://en.41free.com/great-wall-firewall.html
A firewall is mainly the first phase of securing the computer network with an internet and other sources. The elementary reason is to keep away the unwanted sources and requests from the personal computer, which can cause harm to the computer. A firewall can be a hardware device or a software application and normally is sited at the outer shell of the computer network to perform service as the porter for all requests like, incoming and outgoing traffic flow of packets from one source of destination to another. Firewalls are play vital role in computer networks. These are prevent from the viruses and worms which enters from the internet source and affects the internal computer system. So the because of this reason antivirus plays key role in the prevention and securing from the virus. The crucial area is to deliver the meticulous connectivity among the sectors of conflicting conviction levels.
A firewall permits you to begin certain rules to determine what are the in and out requests it consists and which kind of format and whether should be allowed or not into the network packets. With the help of firewalls can restrict the network station i.e. administrator can have the licence to block the some internet information requests packets. By this user cannot access those website which blocked by the TCP/IP in the computer.
 Retrieved on 25 Dec'10 Network Security by, G.Krishnam Raju and SLN Reddy Retrieved by URL: http://www.scribd.com/doc/22594454/ADVANTAGES-OF-FIREWALL
Operational of Firewalls.
 Retrieved on 27 Dec.'10 http://windows.microsoft.com/en-us/windows7/Firewall-frequently-asked-questions
The crucial tenacity of a firewall is to keep the unofficial packets outside the network and to get job done. It allows the authorised and safe information packets to the user. This purpose is able to using the right to use control.
Once the packet is allowed in to the system, then it has to undergo many filtering methods. Once it allowed then packet will be scanned completely to check any harmful virus contains in the packet. If it consists the virus in the packets, then that packet will be discarded. Suppose if packet is safe and not contain any viruses then it'll be allowed further to the user.
In the following figures it showed two types of firewalls, i.e. Internet firewalls where it contains in the internet servers, while accessing the sites, it blocks the harmful sites. Once Internet firewalls allows the packets, then Internal network (fig.2) it checks the and scans the packet again. Then it allows into the computer.
To defining the conditions for access control or reject request differs from one type of firewall to another. Firewalls permit or deny passage based on the type of traffic or on the source or destination addresses and ports. In the above figures which showed Internet network and Internal network two types of firewalls, how the authorized packets are allowing inside and unauthorized packets are blocked by the firewalls.
The different layers of the network.
Retrieved on 2 Jan'10 by, www.comptechdoc.org/independent/networking/protocol/protlayers.html -
In the internet network there are two kinds of layer model, i.e. Open system Interconnection (OSI) and TCP/IP. Both supports the seven layers in the OS. The firewalls have designed in a such a way that they work on respected layers which are assigned individually. The product of firewalls that operate at the minor layers in the OSI model. Such firewalls works high performance with the layers which are in lower throughput layers substances. Because of lower layers, these applications will never checks the actual data is being tranmitted, due to this reason the OSI layers which operates in the lower layers. In the both layers which performs or permits the standard application prototype and rule to perform the services. In the TCP/IP protocol the most adopted layers in the OSI is the DoD which is the standard layers of the OSI protocol stack. Due to this it has been shown only four layers, instead of seven layers. The below diagram shows the OSI and TCP/IP protocol seven layers.
Sorts of Firewalls  
There are four types of firewalls. Which they have unique service qualities for respected grind. Which showed below.
 Retrieved on 30 Dec'10, http://netsecurity.about.com/od/hackertools/a/aa072004_2.htm
 Retrieved on 30 Dec'10.Network Security by, G.Krishnam Raju and SLN Reddy Retrieved by URL: http://www.scribd.com/doc/22594454/ADVANTAGES-OF-FIREWALL
It manages the traffic which enters and exit from the packet with the help of the rules are provided by the OSI layers. The packet which filters and assess the source destinations, packet address and packet originate information's. Due to this the layers which predicts the packet should allowed entering or discarded. It filters all packets before entering into the destination path.
Circuit level gateway
The blocks which consists of information of packets, which enters into the network destinations. To enter the network it has to undergo queuing. The circuit level gateway which performs the gatekeeper works, allows them one after the another in queue inside the source destinations.
To enhance the performance of the network Proxy server is placed. It boosts the performance speed and removes the conjunction from the network . It hides the information's like Ip address, port number of the packet, and serves like the packet has originated from proxy server and it consists the cache server, which stores the information's about the packets. If User 1 wants to enter facebook.com the proxy server sends the request to facebook.com and fetches the home page. If suppose user 2 then connects to facebook.com the proxy server just it shows the message like it has already fetched the information for the user 1 and it returns to the back end and performs the new connections. It performs all such operation's in fraction of a second. Proxy server is used for to block the some sites and also used for the filteration.
It works and performs like the proxy server. The client initially should connects the connection through the application gateway. It checks the packet information, if it authenticated and harmless then it allowed the for the further step. It regulates the flow of packets in the network and performs the process whether to allow the packet or discard the packets. The application gateway performs the all rush-hour traffic of packets against its with the some certain rules and regulations, whether the packets should be allowed or discarded. With the help of the proxy servers the application gateway which performs the high level filtration method.
Proxy server which performs the operations between the client and server. For Example In below showed Figure 4, an application layer firewall is called a "Dual Homed Gateway". It is the dualhomed application gateway; t has two server gateways i.e. Internet gateway and Protected network gateway. These both are connected to the user computer. The proxy server which generates the proxy packets and that will be monitored by the application gateway server. It determines the packet is safe or not, if it consists any viruses or harmful contains, then it tries to delete those viruses. If the viruses counts are in huge numbers then application gateway will discard the packet from the network.
Advantages of the Application gateway:
The application gateway performs the minute filtration's at the entry level of the network, almost 98% of the packets are free from the viruses.
The packets which contains the file like exe extensions.
The high advantage is that no straight network connections ill be available for he packets to entry into the network, all packets should undergo all scanning and filtration levels.
Disadvantages of Application gateway layer:
The main disadvantages is the proxy servers are required in the numerous amount in the network to filter, it takes time consuming and network speed becomes slow.
To write the proxy application programs are difficult and required the skilled labours and huge expensive.
Sample Examples of Firewalls:
The CISCO  company came up with the 500 series of firewalls, which is high in performance and contains the many advance technology programs. It blocks the unsecured packets from the network. It checks the even minute and fractions of virus scans. The main advantage is the, 500 series firewalls keeps the current and past history of the transactions as it contains the ACL. This helps for the company to keep the track. Such high configured firewalls which performs the biometric scanning process. It takes the figure print scan of the packets like IP address, port number, destination and originate address and information about source content. Whenever the packets tries to enter the network then the firewall which scans the packets and performs the biometric test. Which checks the previous history of the packets with the database, if it is authorised and harmless then firewall allows to enter into the network else it discards the packet from the network. The cut-through protocol and ACL which boosts the network speed and enhances the performances.
The CISCO has the major two types of firewalls implemented (IOS firewall and Private Internet Exchange (PIX)) ,which performes the many operations like sharing the files and etc. All both are the filtering types of firewalls, which performs the filtering and blocks the unwanted information's those are harmful for the syste dam.It inspects the all the information's and data which packet consist and tries to remove the harmful content.
For example, the inspection algorithm could determine that a FTP client will need a port opened, changing the filtering rules appropriately. These firewalls system gave the high security system to the systems and also to the network. As the cisco company which works on high level configuration network.
Retrieved on 4 Jan'11. A History and Survey of Network Firewalls [white paper], by KENNETH INGHAM and STEPHANIE FORREST
Another example of firewall with the Wood Grove company.
The Wood Grave company is the financial organization, which has set up the firewalls for the company. The purpose is to set up the firewalls, as the company based on the financial, the more online transactions to be made. Due to this reason the high level security is required.
Wood grove Bank is an dynamic financial organizations domain which provides the group policy for the people. The company uses the windows platform operating system. The company controllers to host the DNS servers to host the website transactions. Discrete PC's congregation Windows Internet Name Service (WINS) for network basic operational purposes. In the company some of the PC's are operating on the UNIX platform operating system which has the Dynamic Host Configuration Protocol (DHCP) services which provides the automatic IP address of the sites
The Wood Grave company is now upgrading with the configuration of the computers. They upgraded the configurations from Windows XP and 2003 with Windows service pack (SP1) to Windows 7, which is latest operating system. When the higher configuration implements the security level also should be increased. With the Windows 7 and Windows service pack (SP2) is using by the company. As the multinational company the online transactions will be more. To secure the transaction organization has set up the firewalls. This allows the safe and harmless packets in the network. Which keeps the history of the online transaction, by this auditing can be done easily.
Lucent's firewall system 
Lucent's firewall is created on packet filtering with state-run. It works same like cisco system where it performs the same operations like blocking the unwanted packets and filtering the contents. Unlike the Cisco products, the firewall can be a bridging firewall.
Retrieved on 4 Jan'11. A History and Survey of Network Firewalls [white paper], by KENNETH INGHAM and STEPHANIE FORREST
Advantages of Firewalls:
The Protocol purifying, the firewalls network which doesn't allows the packets easily. The packets should undergo many filtering process
The main purpose is to hiding the information about the packet information. By hiding the IP address and all information, can be secured from the outside hackers.
The application gateways which is the first step of the filtering process. It scans the packet and filters the contains. Before connecting to the internet the application gateway connects to the firewall network initially
Firewalls controls the traffic network. It allows the packets in the queuing method.
Disadvantages of Firewalls:
By implementing more firewalls system in the system, the network speed will be reduced, and chances of hampering the network.
Expensive to purchase
All hosts should be updated regularly
Difficult to implement.
The Firewalls which plays a vital role in the networking system. It acts like doorkeeper for the unauthorised information's. In the internet so many unwanted and which cause harmful to computer files will be their. By which computer system will get hanged up and some major causes may happen, like hard drive failure, system files will get corrupt etc. To avoid all this and to safer side the firewalls will be implemented in the computer.
It blocks the unwanted packet information which contains the virus or any harmful containt. With the help of firewalls some illegal and unwanted sites can be blocked. In the companies with the firewalls they block the some certain sites to access. New version of firewalls are having the advantages like storing the previous history of the transaction. Due to this it helps for the auditing and check the account status.