Firewall

        Firewall is a system designed to prevent unauthorized access to or from a private network. They can be implemented in both hardware and software, or a combination of both. Firewalls are frequently used to prevent unauthorized Internet users from accessing private networks connected to the Internet, especially intranets. All messages entering or leaving the intranet pass through the firewall, which examines each message and blocks those that do not meet the specified security criteria.

There are several types of firewall techniques:

1. Packet filters: Looks at each packet entering or leaving the network and accepts or rejects it based on user-defined rules.Packet filtering is fairly effective and transparent to users, but it is difficult to configure. In addition, it is susceptible to IP spoofing.
2. Application gateway: Applies security mechanisms to specific applications, such as FTP and Telnet servers. This is very effective, but can impose performance degradation.
3. Circuit-level gateway: Applies security mechanisms when a TCP or UDP connection is established.Once the connection has been made, packets can flow between the hosts without further checking.
4. Proxy server: Intercepts all messages entering and leaving the network. The proxy server effectively hides the true network addresses.

Network security usually is thought of in terms of securing your network against threats that originate from the Internet. Attacks that come from the Internet are common and relatively easy. The Internet was designed to be an open, free flowing system that encourages the unrestricted exchange of information. The Internet was not designed as a secure system that regulates information exchange. On top of the security problem inherent to the Internet is the fact that most TCP/IP based services are also not designed to provide their own security. In order to secure Internet services such as FTP or HTTP, administrators must put into place additional security methods. Despite these risks, the Internet is not the most common source for network attacks. The widespread distribution of hacking information on the Internet has allowed disgruntled or malicious employees to exploit the same vulnerabilities mentioned above on their own networks with little or no security in their way. That's the bad news. The good news is that the same methods used to protect your network from the Internet can be used to protect your network from itself. Implementing multiple DMZ's, strong authentication and digital certificates can help you protect your network (from within and without) as well as provide a more secure opportunity to increase your level of service. With strong authentication, for example, you can make sure that a user authentication attempt originates from a valid source. This also gives you a more secure opportunity to offer remote access into your network from business partners and/or remote employees. The first step toward network security starts with a firewall. After the firewall has been properly installed then other security measures can be more suitably put into place. There are no guarantees in any type of security (network or otherwise). So, if you have extremely sensitive information to protect, then the system storing that information should not be connected to any network (a pair of wire cutters is your best bet for network security). In all other cases, implementing a firewall (or multiple firewalls) is essential to protecting your network. In non-computer industries, a firewall is a specially designed wall that controls the spreading of a fire. In networking, a firewall could be described as a specially designed device that controls the spreading of a network threat. The most commonly talked about source of network threats is the Internet. The Internet is the home of many unknown people that we cannot trust. There are hackers on the Internet that may want to do our networks harm. We can use a firewall to impede an untrusted person from doing damage to our networks.
A more textbook definition of a computer firewall is that it is a method or device that regulates the level of trust between two or more networks. A firewall can consist of software, hardware or a combination of both. A firewall can protect your network from the Internet as well as regulate the traffic between networks within the same company.

For instance, a firewall can allow the legal department's network to have access to the marketing file server but the marketing department can be refused access to legal. In this example the firewall is positioned between the marketing and legal networks so that all communication must pass through the firewall. The firewall is then able to ensure that only authorized packets are allowed.

STATEFUL INSPECTION FIREWALL
A stateful inspection firewall combines aspects of a packet-filtering firewall, a circuit-level gateway, and an application-level gateway. Like a packet-filtering firewall, a stateful inspection firewall operates at the network layer of the OSI model, filtering all incoming and outgoing packets based on source and destination IP addresses and port numbers. A stateful inspection firewall also functions as a circuit-level gateway, determining whether the packets in a session are appropriate. For example, a stateful inspection firewall verifies that SYN and ACK flags and sequence numbers are logical.

stateful inspection firewalls, like all firewalls are not 100 percent effective. So why bother implementing a firewall at all? You should implement a firewall for the same reason you protect your home by locking your doors, despite the fact that this safely measure does not guarantee that an intruder cannot enter your house. Leaving an Internet or intranet connection without a firewall is a careless, open invitation to would-be intruders.