Palo Alto Networks is a start-up with a big goal: replacing traditional network-layer perimeter firewalls altogether.

The company expects most customers will first install its PA-4000 series, next-generation firewalls to supplement their existing firewalls. Then, as users come to trust Palo Alto over time, they will swap out their old firewalls.

PA-4000 devices perform deep packet inspection on traffic originating in business networks that is perhaps destined for servers outside the company. The devices identify what applications are running on the network and apply filters based on them.

Traditional firewalls from Check Point Software, Cisco and Juniper Networks identify applications by the protocols and ports they use, so they cannot distinguish among the many Web applications running through ports 80 and 443, says Rob Whiteley, an analyst with Forrester Research. The Palo Alto gear can distinguish particular applications within Web traffic and filter them.

PA-4000 appliances, for example, can distinguish between Yahoo Mail and corporate e-mail and allow both but block attachments from the Yahoo Mail, Palo Alto says.

Traditional firewall vendors lash together their firewalls and intrusion-prevention systems (IPS) in single devices to offer features similar to those in PA-4000s, says Greg Young, a research vice president with Gartner. These products are not truly integrated, however, he says. Rather, the firewalls and IPSs within these devices pass traffic back and forth and perform their separate functions.

Palo Alto gear can proxy SSL traffic, terminating and decrypting sessions so the content can be inspected and filtered. Traditional firewalls and IPSs that don’t decrypt SSL have no way of screening the content. “IPSs and firewalls are blind to SSL,” says Young. “And SSL traffic is increasing.”