A port number identifies which service on a host should receive network traffic. The IP address gets data to the right machine; the port gets it to the right program. Well-known ports (0 to 1023) are assigned to standard services, and Security+ expects you to recognize the important ones instantly. Ports matter for security because every open port is a potential entry point: firewall rules, scan results, log entries and hardening tasks are all written in terms of ports, so reading them fluently is a practical skill as well as an exam one.
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