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What Is Ethernet?
Learn what Ethernet is, how IEEE 802.3 defines wired LAN communication, how switches use MAC addresses, Ethernet media, speeds, and CSMA/CD.
Ethernet is a family of wired networking standards used to connect devices across a local area network (LAN). It is not the name of one particular cable. Ethernet defines how compatible devices exchange data over copper, fiber-optic, and some legacy media.
An Ethernet LAN normally includes endpoint devices, Ethernet network interfaces, switches, and compatible links. A home computer connected to a router's LAN port, or office computers connected to an access switch, are examples of Ethernet connections.
Ethernet and IEEE 802.3
The Institute of Electrical and Electronics Engineers (IEEE) develops many networking standards. The Ethernet standards family is known as IEEE 802.3.
IEEE 802.3 describes characteristics at two important layers:
- Physical layer: The electrical or optical signals, media, connectors, interfaces, data rates, and maximum link distances.
- Data-link layer: The Ethernet frame format, MAC addressing, and rules for placing frames onto the medium.
Different 802.3 standards support different combinations of speed, medium, signaling, distance, and hardware. Therefore, two devices can communicate directly only when their interfaces and link characteristics are compatible.
What Ethernet Standards Define
Ethernet standards specify more than the shape of a connector or the type of cable. They define the requirements that allow equipment from different manufacturers to work together.
- Copper, coaxial, and fiber-optic media options
- Electrical and optical signaling methods
- Connectors and physical interfaces
- Supported data rates, such as 100 Mbps or 1 Gbps
- Maximum cable or link distances
- Ethernet frame structure and fields
- Rules for sending data onto a network medium
- Compatibility requirements between interfaces, media, and links
A particular Ethernet connection is determined by the standard supported by the interfaces, the installed medium, and the configuration or negotiation between the devices.
Ethernet LAN Components
End devices
End devices are hosts that send or receive network data. Examples include desktop computers, laptops with wired ports, printers, servers, cameras, and other networked equipment.
Ethernet network interfaces
An Ethernet interface, often provided by a network interface card (NIC), connects an end device to the network. The interface converts the device's data into Ethernet signals and processes received Ethernet frames. A built-in Ethernet port is a NIC even when it is not a separate expansion card.
Switches
An Ethernet switch interconnects many Ethernet interfaces. Modern switches are the central devices in most wired LANs. They receive frames on one port and forward them toward the port associated with the destination device.
Links and media
Copper cables, fiber-optic cables, and their connectors form the links between interfaces and switches. The medium must support the Ethernet speed and signaling method being used.
Ethernet Transmission Media
Ethernet has used several physical media over time. The three broad categories below have different costs, distances, and deployment roles.
Coaxial cable
Coaxial cable has a central conductor, insulation, shielding, and an outer covering. It was used by early Ethernet implementations, including shared-medium networks. Most modern office and home Ethernet access connections use twisted-pair copper instead.
Twisted-pair copper
Twisted-pair cable contains pairs of insulated copper conductors twisted together. It is a common choice for short-distance connections from computers and other devices to switches. Its relatively low cost and simple installation make it widespread in homes, offices, and small-office/home-office (SOHO) networks.
Fiber-optic cable
Fiber-optic cable carries information as pulses of light. Fiber generally costs more than copper, but it is useful for longer links, high bandwidth, and connections between wiring areas or buildings. The required fiber type and transceiver must match the Ethernet interfaces.
How Ethernet Delivers Data
Ethernet sends data across a link in units called Ethernet frames. A frame is a Layer 2 data unit. It includes, among other fields, a source MAC address and a destination MAC address.
A MAC address is a link-layer hardware address used for local delivery. When a computer sends data to another device on the same Ethernet LAN, the frame identifies the local destination with the destination device's MAC address.
How a switch forwards a frame
- The switch receives a frame on an interface.
- It learns the source MAC address by recording which switch port received the frame.
- It checks its MAC address table for the destination MAC address.
- If the destination is known, the switch forwards the frame through the associated port. If it is unknown, the switch may send the frame out relevant ports except the port where it arrived.
- The destination interface accepts the frame if the destination address matches it or the frame is otherwise intended for it.
Ethernet frame delivery is local to a LAN or Layer 2 network. IP routing is different: a router uses network-layer IP information to move packets between separate networks. A switch normally forwards Ethernet frames, while a router connects different IP networks.
CSMA/CD and Ethernet Medium Access
CSMA/CD expands to Carrier Sense Multiple Access with Collision Detection. It was the access method associated with shared, half-duplex Ethernet.
- Carrier sense: A device listens to determine whether the medium appears busy.
- Multiple access: Several devices share the same medium and may need to transmit.
- Collision detection: A transmitting device checks whether another transmission interfered with its signal.
On a shared half-duplex segment, two devices could listen at nearly the same time, both decide that the medium was available, and transmit together. Their signals would collide. Devices detected the collision, stopped transmitting, waited for a calculated interval, and tried again. Collisions reduced useful performance as more devices competed for the medium.
CSMA/CD is primarily a legacy or shared-medium concept. Normal modern Ethernet switch links are dedicated full-duplex connections, so a device can send and receive simultaneously and collisions do not occur on that link. CSMA/CD is therefore not used for ordinary full-duplex switched Ethernet operation.
Ethernet Speeds and Evolution
Ethernet is not one fixed-speed technology. Its standards have evolved from early low-speed implementations to high-capacity links used between switches and in data centers.
Bandwidth is the data-carrying capacity of a link. It is commonly measured in bits per second. Mbps means megabits per second, while Gbps means gigabits per second.
The actual speed depends on the Ethernet standard, the capabilities of both interfaces, and the installed copper or fiber. A high-speed switch cannot make an older NIC or unsuitable cable operate at that speed.
Ethernet, LANs, Cables, and Wireless Networking
- Ethernet versus LAN: Ethernet is a technology and standards family. A LAN is a local network covering a limited area, such as a home, office, or campus area. An Ethernet LAN is a LAN that uses Ethernet devices, links, and rules.
- Ethernet versus a cable: Ethernet includes frame formats, addressing, signaling, speeds, interfaces, and access rules. A cable is only the physical medium for one part of an Ethernet link.
- Ethernet switching versus wireless networking: Ethernet switching uses wired Ethernet links and switches. Wireless networking uses radio communication, commonly through wireless access points. Wired and wireless devices can still participate in the same broader LAN and communicate through a switch or other network equipment.
Practical Ethernet Examples
Home or small-office network
A desktop computer connects through a twisted-pair Ethernet cable to a small switch or a router's LAN port. The Ethernet link provides local connectivity to other wired devices. The router may also provide access to other IP networks, including the internet.
Office access network
Employee computers use copper Ethernet links to connect to an access switch. The switch learns their MAC addresses and forwards frames between local devices according to the destination MAC address.
Building or campus uplink
Two switches in separate areas may use a fiber-optic Ethernet uplink. Fiber is appropriate when the distance exceeds typical copper-link limits or when a higher-capacity connection is needed.
Legacy and modern Ethernet compared
In a legacy shared half-duplex segment, multiple devices contend for the same medium and CSMA/CD handles collisions. In a modern switched LAN, a workstation normally has a dedicated full-duplex link to a switch, allowing simultaneous sending and receiving without collision detection.
Troubleshooting Ethernet Problems
No wired network connection
- Verify that the Ethernet cable is connected at both ends.
- Check the link-status indicators on the device interface and switch port.
- Confirm that the device, switch port, and copper cable or fiber type are compatible.
- Test with a known-good cable or switch port when appropriate.
Link speed is lower than expected
- Check the Ethernet capabilities of both interfaces.
- Verify that the installed copper category or fiber type supports the intended standard and distance.
- Check for speed and duplex negotiation or configuration mismatches.
- Remember that the link operates at a speed supported by the complete path, not merely by the fastest device.
Poor performance on a legacy half-duplex link
- Consider whether collisions are occurring on a shared medium.
- Inspect for a duplex mismatch, where one side uses half duplex and the other uses full duplex.
- Check for inappropriate shared-media equipment or cabling.
- Contrast the result with a dedicated full-duplex switch connection, where collisions should not be a normal part of operation.
Exam-Relevant Summary
- Ethernet is a family of wired local-area networking standards, not a single cable type.
- IEEE 802.3 is the standards family associated with Ethernet.
- 802.3 covers physical-layer details and data-link-layer behavior.
- Ethernet frames use source and destination MAC addresses for local delivery.
- Switches forward frames using learned MAC address information.
- Routing between separate networks is an IP and router function, not ordinary Layer 2 switching.
- CSMA/CD applies mainly to shared half-duplex Ethernet and is not used on normal full-duplex switched links.
- Ethernet supports many speeds, including 10 Mbps, 100 Mbps, 1 Gbps, 10 Gbps, and 100 Gbps.
- Twisted-pair copper is common for short access links; fiber is common for longer or higher-capacity links; coaxial Ethernet is largely legacy.
For a connected overview of the subject, see What Is Ethernet?.