Firewall
Firewalls play an important role in restricting and controlling access to networks. A firewall is normally implemented within a router or gateway, and will monitor incoming and outgoing traffic at the boundary of the protected zone. It is a device that denies external hosts access to selected insecure services within the protected zone, while also denying internal hosts access to insecure services outside the protected zone .
Types of firewall:
1. The Application-Proxy Firewall: In a proxying firewall, every packet is stopped at the firewall. The packet is then examined and compared to the rules configured into the firewall.
2. The Application-Gateway Firewall: Application-gateway firewalls also operate on Layer 7 of the OSI model. Application-gateway firewalls exist for only a few network applications. A typical application-gateway firewall is a system where you must telnet to one system in order telnet again to a system outside of the network .
V.S.Bagad, I.A.Dhotre Computer Networks, second revised edition, 2009