 Welcome to Relationships Among Firewall Perimeters. When we use the term firewall, we refer to a device or application used to isolate our network from other networks. Just as a physical firewall in a multifamily dwelling prevents a fire from moving from one home to another, a network firewall prevents unauthorized users or messages from moving from one network or operating system to another. The firewall blocks the intrusion of software that has been specifically designed to damage or disrupt an operating system or network. This type of software is called malware, which is short for malicious software. Some examples of malware include viruses and trojans. It's important to understand that firewalls can be implemented as hardware, software, or as a combination of the two. Applications can run on dedicated firewall devices, servers, or clients. A strong network defense strategy would include all of these options. Network classifications. Trusted networks are inside the network security perimeter and the networks you're trying to protect. Semi-trusted networks may allow access to some database materials in email, not be used for confidential or proprietary information, and include DNS proxy and modem servers. Untrusted networks are referred to as the demilitarized zone, or DMZ, yet are often anything but outside of your security perimeter and outside of your control. You have completed this learning activity, relationships among firewall perimeters.