Network Firewall Implementation

By Edward Tetz from Cisco Networking All-in-One For Dummies

A strong perimeter security helps to protect your network from external attacks. The main element on the perimeter security front is a network firewall. You can deploy several types of firewalls and other security options. The different types of network firewalls include the following:

Packet filtering: These firewalls use ACLs to inspect the data that they forward down to the IP layer. This inspection allows them to classify data based on the TCP or UDP ports, as well as the source and destination IP addresses. This filtering allows you to make forwarding decisions. Some organizations use packet filtering to allow only traffic that meets approved criteria to pass out of the firewall.
Stateful inspection: Also known as Stateful Packet Inspection (SPI) firewalls, these firewalls not only allow packet filtering, but it pays attention to the flow of the packets. Rather than evaluating each packet as a separate entity, it looks at the flow of the traffic and identifies packets that are replies to others. SPI can evaluate packets that are suspicious and part of an attack profile.
Application layer firewall: This firewall can be a specific firewall, but it tends to fall in the category of proxy and reverse proxy servers. In this case, there can be a deep packet inspection into the data to validate that it is not only allowed, but also not part of an attack on the systems that make up your network.

These firewalls tend to be specific for the application layer protocol that they are protecting. Common choices here are HTTP, FTP, and SMTP.