ABSTRACT
This chapter investigates some of the devices that are used at the Internet and transport layers of the Internet model. The main devices are as follows:
• Stateless/Packet-filter firewalls. These are typically routers that filter data packets at the Internet (network) and transport layers, and thus filter network addresses and TCP/UDP ports. These are typically known as screening firewalls. These devices tend to be stateless, where each packet is looked at independently from all others.
