A firewall is a system that makes network fully secure. It works by filtering incoming and outgoing traffic to a network driven by user-defined rules. The key purpose of a firewall is to diminish or completely remove unwanted network communications while allowing flow of genuine communication. Firewall play a key role in all types of server infrastructures as it helps in creating an additional and much-needed layer of security. When combined with other security measures, firewall can play a major role in preventing unauthorized access to your system. There are many types of firewalls available. Packet filtering is one of the most commonly used systems as it provides extremely high levels of security. It is effective and transparent too but there are some drawbacks such as configuring issues and susceptibility to IP spoofing. Another commonly used firewall is the circuit level gateway implementation. It is a mechanism that triggers security measures when a UPD or TCP mechanism is established. Web application firewall is another type of firewall system. It is a hardware appliance that can be customized to meet the specific security requirements of a server system. Proxy servers are gateways that stay behind the actual network address of the computers. It slows down the network to control the traffic and check unauthorized access.