A firewalls is a network security device that monitors and regulates network traffic based on predefined security rules. It forms a vital part of the network security system separating the trusted network from an untrusted network, such as the Internet. 

Second-Generation firewalls, also known as stateful firewalls, not only filter packets but also keep track of particular communications between endpoints by remembering which port number the two IP addresses use for their connection at layer 4 (transport layer) of the OSI model. So that these firewalls allow examination of the overall exchange between the nodes.

Next generation firewalls provide application/Layer 7 filtering. The main advantage of application layer filtering is that it can detect certain applications and protocols. This feature allows next generation firewalls to detect if a permitted protocol is being exploited or to identify undesired applications or services using a non-standard port.