DOI link for Network Firewalls
Network Firewalls book
Abstract Firewalls are network devices that help enforce an organization’s security policy. Since their development, various methods have been used to implement ﬁrewalls. These methods ﬁlter network trafﬁc at one or more of the seven layers of the ISO network model, most commonly at the application, transport, network, and data-link levels. Newer methods, which have not yet been widely adopted, include protocol normalization and distributed ﬁrewalls.