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"A firewall is a part of a computer system or network that is designed to block unauthorized access while permitting authorized communications. It is a device or set of devices that is configured to permit or deny network transmissions based upon a set of rules and other criteria". 
Figure 1: Firewall 
The term firewall came into being used from the shield that prevents from fire. That acts as a barrier for danger. Later on the term began to use for computer security when the first Internet attack was delivered which is known as morris worm. Before this no one has even thought such kind of attacks that could be proved as harm for the computing world. Then a stream of security search began which gave the birth to computer firewalls. Different kinds of firewalls began to develop for different purposes. 
In the start the firewall were being used in the computer machines only. But later on when the firewall became famous and get progressed, they began to implement in the hardware. For better performance, the firewalls were implemented in hardware devices. Thus the hardware was much reliable and viable. 
The first paper on firewall technology was published in 1988. There are different firewalls technologies are being used for different purposes. Their implementation depends on the environment. The major implementation of firewall started from private businesses. The main reason was that the Internet was no commonly being used but there was a high implementation of computer machines in private businesses. The huge organizations can afford the high cost of computer applications. So, the firewalls were not affordable for the home use clients.
Types of Firewalls:
There are different types of firewalls as
Packet filter was written by Daniel Harmeier. This was the start of firewall technology. This is also called the first generation of firewalls. Packet filter firewalls, inspects each and every packet that comes. All the packets are checked according to user define rules. On the bases of these rules packets can be accepted or rejected. In some cases there is a notification send to the active user regarding to the action., [3,  After the configuration of user the action is taken otherwise the packet is send to block list which prevents the packet to take action., 
Its working includes the Network Address Translation and Quality of service. Network address translation includes the translation of IP address into hardware address and then again the hardware address into IP address. These two translations continue as a recursive functional dependencies. There are also different other interfaces implemented in the packet filter firewall which serve different purposes as plog which served the packet transformation to carry the packets from kernel to the application level. In the application layer the user can control over the packets for further manipulations. These firewalls are compatibles with the Berkely Software Distribution's and Microsoft windows also , , , .
Packets filter firewall work on the first three OSI layer. It means that they work on the physical, data link layer and network layer. From physical layer all the packets are being captures for processing, later on data link layer, these packets are checked according to the data inside each and every packet. Then on the network layer, the permissions are checked as there network address translation is carried out. The IP addresses and port addresses are translated. If there is a restriction of telnet network, then the port number 23 which is dedicated to telnet, will be blocked from packet filtering., 
These are some criteria on the bases of those the packets are being drops or allowed on a network.
IP addresses of source and destination.
ports of source and destination.
different types of flags in the header section of packets.
The most IP packet filter firewalls are stateless which means that they cannot remember information of previous packet processed. But there are packet filters with state which know the state of previous packet. This is very helpful in the situations where there is a stream of packets, so, there is no need to check each and every packet each and every time. We can check the first packet of that stream and then allow the other packets to pass on the bases of previously checked packet. 
Application firewalls are also called the second generation firewalls. This type of firewall are basically server the purpose of a barrier between application layer and input, output and processes. It controls the input, output and different processes for application sections. It the controls are according to the firewall configuration, then ok. Otherwise, the control is blocked. There are two further division of application firewalls network-base application firewall and host-base application firewall. 
Network-base application layers are the computer network firewalls that run for the application layer over network stack. Network-base application firewalls are also known as the proxy-base firewalls. They are specific to the particular traffic. They can be implemented as software running on hardware or a separate hardware that can be installed. They are the intermediate between protocol stack and application layer. 
Host-base application layer also embed the features of network-base application firewalls. They serve the same purpose of network-base application firewalls. But the additional feature is that the host-base application firewall runs for the same hosts. It means that, the communication between different hosts is controlled by the host-base application firewall. There are database application firewalls which control over the attacks of different applications on databases. 
Stateful firewalls are also known as the third generation firewalls . This type of firewalls keeps track of the connections and then performs the actions. It keeps the information about the type of connection and then allows or rejects the packets. The firewall picks the packet and then configures the source of connection for packets. After that it will take the decision according to firewall rules that weather the packets are allowed or rejected to a connection or not. The firewall works on the third layer of OSI which is network layer three. This type of firewalls required high processing speed. The checking of a packet that weather it belongs to the authorized connection or not is done by validating the header of packets. So, there is a need to check each and every packet that entered in the IP stack. 
There is also a need to speed up the processing, for this purpose the information about the connection is stored in the temporary memory. The information of the first packet's connection is stored after extracting the attributes. For the next one, the attributes or properties are not extracted again, but we just recognize the connection of packet and then we test all the previously stored information for that particular connection. Thus we have an opportunity to test secure our resources. To establish a new connection, the first packet is send to the server with some synchronous bits that carries the information about the connection. If the services are available, the connection is set otherwise rejected. 
This type of firewall is opposite to the statesful firewall. In contrast to the stateful firewalls they do not remember the state of packets or session created. So, they do not need any memory to store the information about packets. They need not memory to contain the state of packets. They operate like memory less machines or toggle switch that is not concerned to the previous state but just the current one that is known. They are much faster than the stateful firewalls due to record base. , 
Stateless firewalls receive, block or reject the packets on the bases of very simple rules as the header address or other static values etc. It has no way to know about the type of connection or other malicious actions that can be carried out by Internet. , 
This type of firewalls just controls over the validation of a session. They do not check the all packets, but just the first packet and then matches their connections. On the bases of those the communication is allowed or restricted. 
They are similar to the packet filtering firewalls but the difference is just of those circuit-level firewalls works on the network layer and session layer in OSI model. It checks the Transmission Control Protocols or User Datagram Protocols and then create the sessions if the connection is valid otherwise it down the connection. 
There are some packet states checking which circuit-level firewalls checks as
IP addresses of source and receiver
Port addresses of source and receiver
Firewalls also secure the data from unauthorized entities. For this purposes different types of encryption techniques are also used.
Network-level firewalls were introduced in 1998.  These firewalls are the one of fast firewalls. They are heavily using in network-base applications. They are providing the services even in the intelligent appliances as routers. They support the cipher techniques as encryption and decryption. They can deal security level applications. They do not understand the formal web development languages so, they cannot take notice about the input coming in the form of web requests from client etc. this leakage of security can cause of serious attacks that can prove harmful for network. They were building on the working of packet filter firewalls. Contents of packet filter firewalls are extended in network-level firewalls as headers and payload encryption etc. they are especially for networks due to their agility. Their processing capability is very robust. So, that's why they are very popular for network-base infrastructures. , 
The acceptance and rejection in such type of fire walls are the headers and ports. The user can restrict certain IP addresses to access the computer or can perform any other operation on a certain IP addresses. So, those IP's will be not entertained. Similarly, for certain ports can also be checked as unavailable for remote user. So, those will be restricted by network-level firewalls for remote users. They work like a check points that just allow passing the packets that have the permissions or reject the packets that have no authority to get entrance in the computer networks. , 
In the future they, there is a bright future for network-level firewalls and application firewalls. The both can be combined for the better results. Application firewalls will be used to control over the actions of the applications for Internet while network-base firewall will be used for speed and performing the actions of acceptance or rejection of packets identified by application firewall. So, the overall security of networks will be shape up. 
Firewalls are the important tools that play a vital role in computer security. There are different types of firewalls as
Types of firewalls
Packet Filter Firewalls
Operates on the packets
Application Layer Firewalls
Operates on the application and kernel space
Operates on the bases of previously monitored packets
Operates on the bases of no record of monitored packets
Operates on the validation of sessions
Operates on network-base applications
Firewalls are the important devices which collaborate each other for secure communications. There are different types of firewalls that serve different purposes. They all, by joining perform a secure network. It's very necessary for a secure communication that all the firewalls must the united so that an optimized level of security can be achieved. As in computer there are different applications are running at same time because now a days the computer are not job specific. So, a single firewall is not enough for secure communication. There is a need of compound firewalls that can serve the purpose of security and integrity for computer networks. There is also a need to handshake different firewalls because it's very difficult to rescue the networks from being vulnerable.