Ethernet
The LAN technology: CSMA/CD history, switching present, 802.3 framing.
Level: beginner
What is it?
Ethernet is a family of wired local-network technologies standardized mainly under IEEE 802.3. Modern switched full-duplex Ethernet links use frames and MAC addressing, while collision detection belongs primarily to the historical shared-medium/half-duplex model.
Why does it matter?
Ethernet provides a common link-layer framing and delivery technology for local networks. Understanding frame boundaries, MAC addresses, VLAN tagging, and switches helps diagnose local connectivity independently of IP routing.
Mental model
An Ethernet frame is the envelope used for one local-link hop. A switch learns which MAC addresses are reachable through its ports and forwards known unicast frames toward the appropriate port.
How it works
- A frame contains destination and source MAC addresses, a type/length field, payload, and frame-check sequence.
- Switches learn source MAC addresses and associate them with ingress ports in a forwarding database.
- Unknown unicast and broadcast frames may be flooded within a VLAN, subject to switch behavior and policy.
- VLANs divide a physical switching fabric into logically separate layer-2 broadcast domains.
- Full-duplex switched Ethernet avoids the collisions described by classic CSMA/CD shared-medium Ethernet.
Steps
- Host A sends a frame addressed to Host B's MAC address.
- The switch records A's source MAC on the incoming port.
- If B's MAC is known in the same VLAN, the switch forwards only to B's port.
- If unknown, the switch may flood the frame within the VLAN and learn B when it replies.
Real-world usage
- Troubleshooting link state, VLAN membership, and unexpected broadcasts.
- Reading packet captures and switch forwarding tables.
- Distinguishing local Ethernet forwarding from IP routing.
Common mistakes
- Saying modern switched full-duplex Ethernet still relies on collision detection in the same way as shared coaxial Ethernet.
- Assuming a switch routes traffic between IP subnets without a layer-3 function.
- Confusing MAC addresses with IP addresses or port numbers.
Interview questions
What information does a basic Ethernet switch use to forward frames?
It primarily uses the destination MAC address and its learned forwarding database within the relevant VLAN.
When does an Ethernet switch flood a frame?
For broadcast traffic and often for unknown unicast or multicast according to its configuration and protocol behavior, within the relevant VLAN.