How Cisco Broadband Routers and Firewalls Work

Here's a sample chapter from the Cisco Press Book on Routers and Firewalls...

Broadband Routers and Firewalls > How Broadband Routers and Firewalls Work

How Broadband Routers and Firewalls Work

Many broadband routers and firewalls function primarily through the use of Network Address Translation (NAT) to hide the internal systems behind a single external IP address. These so-called "NAT routers" or "NAT firewalls" do an adequate job of hiding resources from casual attack methods, but they do not perform advanced firewall functions; therefore, it is really a bit of a misnomer to call them firewalls, at least in the sense that firewalls such as the Cisco Secure PIX Firewall, Microsoft ISA Server, and Check Point Firewall-1 products are considered firewalls. Rather, many broadband routers and firewalls are just NAT-based packet-filtering routers providing a degree of privacy, but they typically lack advanced firewall features such as stateful packet inspection (SPI), proxying of data, or deep packet inspection.