 Attacks by Using Firewalls A firewall is a device, software application, or set of devices at carriers, enterprises, and homes that screens incoming and internal traffic to prevent hackers from access- ing files. Firewalls deny hackers access by allowing only designated users to access networks. In an organization's network, firewall software is installed on routers and appliances. For additional protection, organizations that use their carrier's firewall pro- tection have onsite firewall protection, as well. Firewalls provide networks partial protection against Denial-of-Service (DoS) attacks and intrusion. They use various techniques including address filtering, which looks at a user's IP address and accepts or rejects messages based on that address. Important applications might contain their own firewalls for extra protection. Managed Services Communications can also be restricted to certain addresses. Firewalls contain hundreds of ports and can filter on a port-by-port basis. In addition, they can be programmed to recognize applications and content that can be programmed to pass through firewall by using a specific port. However, even with these filtering capabilities DoS attacks can congest access to the organization's files. Firewall ports can be virtual as well as associated with physical equipment within the firewall. A virtual port is programmed to allow certain traffic such as Voice over Internet Protocol (VoIP) calls to pass through the firewall to its destination. Because employees use their laptops at home to surf the Web and then bring them into work, corporations monitor internal transmissions as well as communications from the Internet. The goal is to avoid contamination from either source. Firewalls don't protect against viruses and other threats such as e-mail spam. Cor- porations often subscribe to security services that keep them posted about new soft- ware attacks. The software monitors their networks for unusual amounts or types of traffic and downloads software patches for protection again