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Networking Fundamentals · Foundational

Firewall

A device or software that permits or denies traffic between networks or hosts according to a security policy.

Definition

Firewalls filter on addresses, ports and protocols and, in stateful and next-generation forms, on connection state, applications and users. They are deployed at network edges, between internal zones and on hosts. Default-deny with explicit permits is the standard approach.

Example

A perimeter firewall allows inbound TCP 443 to a web server in the DMZ and blocks everything else.

Exam tip

Firewalls filter; routers route. Many devices do both, but exam answers separate the roles.

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