What Is a Port Number?
a Port Number — a 16-bit number (0–65535) that identifies which application or service traffic belongs to on a device — the apartment number to the IP address's street address.
How it works
Well-known ports (0–1023) mark standard services: 80 HTTP, 443 HTTPS, 22 SSH, 53 DNS, 25 SMTP. Clients use ephemeral high ports for their side of connections. Firewalls, NAT and load balancers all make decisions on ports.
Why it matters
Port fluency is non-negotiable for CCNA, security work and interviews — expect rapid-fire "which port does X use?" questions. Drill them with our practice tests.
Frequently asked questions
What are well-known ports?
Ports 0–1023, reserved for standard services — like 80 (HTTP), 443 (HTTPS), 22 (SSH) and 53 (DNS).
Why do clients use random high ports?
Each outgoing connection needs a unique local endpoint; ephemeral ports (typically 49152–65535) provide that.
Can two services share a port on one IP?
Not on the same protocol — the port identifies exactly one listening service per IP per protocol (TCP/UDP counted separately).
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