A wired Ethernet connection requires several pieces of hardware to work together. The endpoint device needs a network interface card, the cabling must provide a physical path, and networking devices such as switches, routers, modems, optical network terminals, and access points must be connected correctly.
CompTIA A+ technicians should be able to identify these devices, understand their physical connections, interpret link indicators, verify port speeds, and troubleshoot basic network hardware problems.
Network Interface Cards and Ethernet Adapters
A network interface card, commonly called a NIC, allows a computer or other device to connect to a network. A NIC may be built into the motherboard, installed as an expansion card, or connected through an external port such as USB.
The NIC sends and receives network frames, provides the physical Ethernet connection, and identifies the device on the local network through a Media Access Control address.
Onboard Network Interface Cards
An onboard NIC is integrated into the computer’s motherboard. Most modern desktop computers and many laptops include an onboard Ethernet adapter.
The RJ45 Ethernet port is normally located on the rear input/output panel of a desktop motherboard or along the side of a laptop. The onboard NIC may support speeds such as 100 Mbps, 1 Gbps, 2.5 Gbps, 5 Gbps, or 10 Gbps, depending on the hardware.
Onboard NICs are convenient because they do not require an expansion slot or separate adapter. They are also normally recognized automatically by the operating system after the correct driver is installed.
An onboard NIC may be disabled in the BIOS or UEFI firmware. It may also be disabled through the operating system. If the Ethernet port does not appear in the operating system, the technician should verify the firmware setting, device status, and driver installation.
If an onboard NIC fails, the technician may be able to disable it and install a PCIe network card or use a USB Ethernet adapter instead of replacing the entire motherboard.
PCIe Network Interface Cards
A PCI Express NIC is an expansion card installed in a desktop computer’s PCIe slot. It provides one or more physical network ports.
PCIe NICs are commonly used when:
The onboard NIC has failed
A faster network speed is required
Additional Ethernet ports are needed
A fiber connection is required
Specialized networking features are needed
A server requires multiple network connections
Many standard Ethernet NICs use a PCIe x1 slot. Higher-speed adapters, multiport server cards, and fiber network cards may use larger PCIe interfaces.
Before installing a PCIe NIC, the technician should verify that the motherboard has an available compatible slot and that the operating system supports the card.
The technician should shut down the computer, disconnect power, follow electrostatic discharge precautions, insert the card firmly into the slot, secure it to the case, and install the appropriate driver.
Some PCIe NICs contain standard RJ45 ports, while others contain slots for removable fiber or copper transceivers.
USB Ethernet Adapters
A USB Ethernet adapter adds a wired network connection through a USB port. These adapters are commonly used with thin laptops, tablets, and other devices that do not include a built-in RJ45 port.
A typical adapter contains a USB connector on one end and an RJ45 Ethernet port on the other.
USB Ethernet adapters may connect through:
USB-A
USB-C
Thunderbolt-compatible USB-C ports
Docking stations
Port replicators
The adapter’s maximum network speed may be limited by the USB connection, adapter hardware, driver, cable, or network port.
For example, a Gigabit Ethernet adapter may not reach full speed when connected through an older or slower USB interface. A USB-C adapter may support Gigabit or multigigabit Ethernet, but the device and cable must also support the required USB data rate.
USB Ethernet adapters are useful for troubleshooting. A technician can connect a known-good adapter to determine whether a computer’s onboard NIC has failed.
Physical Ethernet Ports
The most common physical copper Ethernet port uses an RJ45-style connector. The port contains eight contacts that connect to the eight conductors inside a twisted-pair Ethernet cable.
The connector’s locking tab helps hold the cable securely in the port. A damaged locking tab may allow the cable to become loose and cause an intermittent connection.
Ethernet ports may be found on:
Desktop and laptop computers
Network switches
Routers
Wireless access points
Printers
VoIP phones
Security cameras
Docking stations
USB adapters
Game consoles
Smart televisions
Some network adapters use fiber-optic ports or removable transceiver modules instead of standard RJ45 ports. The physical connector and transceiver must match the network cable type.
Network Drivers
A network driver allows the operating system to communicate with the NIC. Without the correct driver, the operating system may not recognize the adapter or may provide limited functionality.
A missing or incorrect driver can cause symptoms such as:
Network adapter not appearing
Unknown device in Device Manager
Ethernet port not functioning
Reduced network speed
Frequent disconnections
Missing advanced adapter settings
System instability
Technicians should obtain drivers from the computer manufacturer, motherboard manufacturer, NIC manufacturer, or another trusted source.
After installing or updating a driver, the technician should confirm that the adapter appears correctly in the operating system and does not display an error status.
A driver may occasionally introduce problems. If connectivity fails after a driver update, the technician may need to roll back to the previous version.
Link Speed
Link speed is the data rate negotiated between two directly connected Ethernet devices. For example, a computer NIC and a switch port may negotiate a connection at 100 Mbps, 1 Gbps, 2.5 Gbps, or another supported speed.
The final link speed is normally limited by the slowest component in the physical connection.
The connection depends on:
NIC capability
Switch-port capability
Ethernet cable category
Cable condition
Connector quality
Driver configuration
Auto-negotiation settings
A computer with a 2.5 Gbps NIC will normally connect at only 1 Gbps when attached to a Gigabit switch port.
A damaged cable may cause a Gigabit connection to fall back to 100 Mbps. Gigabit Ethernet normally requires all four wire pairs to be correctly terminated, while some older 100 Mbps connections can operate using fewer pairs.
A technician should check the negotiated link speed in the operating system or switch management interface when investigating slow performance.
Auto-Negotiation
Auto-negotiation allows two Ethernet devices to determine the highest speed and duplex mode supported by both ends.
In most situations, auto-negotiation should remain enabled. Manually configuring one side of the connection while leaving the other side on automatic settings may create a speed or duplex mismatch.
A duplex mismatch can cause poor performance, frame errors, retransmissions, and unreliable connectivity.
When manual configuration is required, both ends of the connection should normally use matching settings.
MAC Addresses
A Media Access Control address, commonly called a MAC address, is a hardware identifier associated with a network interface.
A MAC address is usually written as six groups of hexadecimal characters, such as:
00-1A-2B-3C-4D-5E
Each physical or virtual network adapter may have its own MAC address. A computer with wired Ethernet and Wi-Fi normally has a different MAC address for each adapter.
Switches use MAC addresses to determine which physical port should receive an Ethernet frame. The switch learns which MAC addresses are connected to its ports and stores this information in a MAC address table.
MAC addresses may be used for:
Device identification
DHCP reservations
Access control
Network inventory
Switch-port tracking
Troubleshooting
Wireless filtering
Although MAC addresses are often assigned by the hardware manufacturer, many operating systems and devices support MAC address randomization or software-based address changes. A technician should not assume that a displayed MAC address is permanently fixed.
NIC Status Indicators
Many Ethernet ports include light-emitting diode indicators. These lights may show whether a physical connection exists and whether network activity is occurring.
The exact light behavior varies by manufacturer, but common indicators include:
Solid light for an established link
Flashing light for network activity
Different colors for different link speeds
No light when no connection is detected
A link light confirms that the NIC and connected device have detected a physical connection. It does not guarantee that the device has a valid IP address or can reach the internet.
If no link light appears, the technician should inspect the cable, connectors, NIC, switch port, and device configuration.
Switches and Physical Ethernet Connections
A network switch connects multiple wired devices within a local area network. Each computer, printer, access point, camera, phone, or other device connects to a physical switch port.
The switch receives Ethernet frames and forwards them toward the correct destination based on MAC addresses.
For CompTIA A+, technicians should understand the physical role of the switch and how devices connect to it.
Switch Ports
A switch contains multiple Ethernet ports.