firewall

Firewall Prevent unauthorised access Just explains what it does + not how it does it

First generation packet filtering 1.0 Static packet filtering each packet entering or leaving the network is checked and either passed or rejected - set on user rules Source IP address Destination IP address TCP/UDP source port TCP/UDP destination port

Stateful packet inspection 1.1 This approach examines the contents of packets rather than just filtering them Permit outgoing sessions while denying incoming sessions.

Second-generation firewalls: Proxy services increase the level of security between trusted and untrusted networks. Known as application proxy or gateway firewalls An application gateway firewall uses software to intercept connections for each Internet protocol and to perform security inspection. It involves what is commonly known as proxy services. The proxy acts as an interface between the user on the internal trusted network and the Internet. Each computer communicates with the other by passing all network traffic through the proxy program. The proxy program evaluates data sent from the client and decides which to pass on and which to drop

Firewalls evolved: The third generation state-of-the-art perimeter security These systems  alert administrators in real time about suspicious activity that may be occurring on their systems New name given - stateful multilevel inspection, or SMLI screens the entire packet, OSI layers 2 through 7, and rapidly compares each packet to known bit patterns of friendly packets before deciding whether to pass the traffic