A firewall enforces rules about which network traffic may pass between zones, such as between the internet and an internal network or between internal segments. Firewalls differ mainly in how deeply they look at traffic, and Security+ tests whether you can match the right type to a threat. The layer numbers refer to the OSI model: a Layer 4 firewall decides using addresses, protocols and ports, while a Layer 7 firewall understands the application data itself. Choosing correctly means knowing what each can see and what it cannot.
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