When a user says "the internet is down," the fastest clue is often two tiny lights next to a network jack. Before you open any software, you can look at those LEDs and learn whether the cable is even talking to the switch. Reading them correctly saves time on nearly every wired connectivity call.
CompTIA A+ Core 1 (220-1201) covers Ethernet cabling and physical networking in its Networking domain. The objective expects you to recognize the physical Ethernet interface, understand what its status lights mean, and use those indicators along with cable and connector knowledge to diagnose a wired connection. This article stays on what a technician actually sees at the port: the connector itself, the link light, the activity light, and the physical layer clues they give you.
The Ethernet port is a standardized jack, and knowing its shape matters
The wired network port on a PC, laptop, switch, or router is an RJ45 jack. RJ45 is the common name, though the technically correct connector name is 8P8C, meaning eight positions and eight contacts. That tells you the connector holds eight pins, which map to the four twisted pairs inside a standard Ethernet cable.
The port is keyed so the cable's plastic tab (the retention clip) can only seat one way. When it's fully inserted, you hear or feel a click, and the clip holds the plug against removal. A loose plug is one of the most common physical faults you'll find, because the clip breaks off easily and the plug backs partway out.
Physically, the RJ45 jack is wider than a phone jack, which uses the smaller RJ11 (typically 6P2C or 6P4C). On the exam and on the job, don't confuse them. A phone cable can physically start to fit an Ethernet port but won't seat correctly and won't carry Ethernet.
The wiring inside follows the T568A or T568B pin arrangement. You don't terminate cables during troubleshooting, but you should know that both ends of a straight-through cable must use the same standard. Mismatched or miswired pairs cause a port to fail to link or to negotiate a slower speed than expected.
Link lights tell you the physical connection exists
Most Ethernet ports have two LEDs, one on each side of the jack or stacked together. The first is the link light. Its job is simple: it shows that the port has established a physical link with the device on the other end of the cable.
A steady, lit link light means the local port and the remote port have completed link negotiation. That's a big deal in troubleshooting, because it confirms three things at once: the cable has continuity, both devices are powered, and the two ports agreed on a connection. When the link light is on, the physical layer is healthy even if higher-level networking still fails.
No link light usually points to a physical problem. The cable may be unplugged, damaged, or plugged into a dead port. The far-end device may be off, or the port may be administratively disabled on a managed switch. When a user reports no connectivity and the link light is dark, start with the cable and the switch port, not the operating system.
In exam terms, remember this order: link light off means look at physical layer problems first. Link light on means the wire is fine and you can move up to IP configuration, DHCP, or DNS.
The color of the link light often signals negotiated speed
On many ports, the link LED also encodes the connection speed through its color.