The Dominant Wired Technology
Ethernet is the most widely used wired networking technology. When you plug an Ethernet cable into your laptop, you're using Ethernet. It defines the rules for how data is formatted, transmitted, and received on a local network.
Ethernet operates at both the Physical Layer and the Data Link Layer of the OSI model. It specifies the cable types, connector shapes, signaling methods, and frame format.
Ethernet History
Ethernet was invented by Robert Metcalfe at Xerox PARC in 1973. It has evolved through several generations:
Standard │ Speed │ Cable Type │ Year
─────────────┼─────────────┼────────────────┼──────
Ethernet │ 10 Mbps │ Coaxial │ 1973
Fast Ethernet│ 100 Mbps │ Cat 5 UTP │ 1995
Gigabit │ 1 Gbps │ Cat 5e/6 UTP │ 1999
10 Gigabit │ 10 Gbps │ Cat 6a/7 UTP │ 2002
40 Gigabit │ 40 Gbps │ Fiber │ 2010
100 Gigabit │ 100 Gbps │ Fiber │ 2010
Each generation increased speed while maintaining backward compatibility. Your modern Gigabit Ethernet switch can still work with older 100 Mbps devices.
Ethernet Frame Format
Ethernet frames have a specific structure defined by the IEEE 802.3 standard:
┌────────┬─────────┬─────────┬────────┬─────┬─────┐
│Preamble│Dest MAC │Src MAC │ Type/ │Data │ FCS │
│ 8 bytes│ 6 bytes │ 6 bytes │Length │ │4 B │
│ │ │ │ 2 bytes│ │ │
└────────┴─────────┴─────────┴────────┴─────┴─────┘
- Preamble (7 bytes) — Alternating 1s and 0s that synchronize the receiver.
- SFD (1 byte) — Start Frame Delimiter. Marks the actual beginning of the frame.
- Destination MAC (6 bytes) — The MAC address of the intended recipient.
- Source MAC (6 bytes) — The MAC address of the sender.
- Type/Length (2 bytes) — If > 1536, it's a Type field indicating the protocol (0x0800 = IP, 0x0806 = ARP). If <= 1536, it's a Length field.
- Data (46-1500 bytes) — The actual payload. Minimum 46 bytes (padded if necessary), maximum 1500 bytes (MTU).
- FCS (4 bytes) — Frame Check Sequence. CRC for error detection.
Half-Duplex vs. Full-Duplex
Early Ethernet used half-duplex communication — devices could either send or receive, but not both at the same time. This required CSMA/CD to manage collisions.
Modern Ethernet uses full-duplex with switches — devices can send and receive simultaneously. There are no collisions because each device has its own dedicated connection to the switch. CSMA/CD is no longer needed.
VLANs: Virtual LANs
A VLAN (Virtual LAN) allows you to logically divide a single physical switch into multiple separate networks. Devices on different VLANs can't communicate directly — traffic must pass through a router.
VLANs are useful for security and organization. For example, you might put all employee computers on one VLAN and all guest devices on another, even though they're connected to the same physical switch.
Physical Switch
┌──────────────────────────────────────┐
│ Port 1-8: VLAN 10 (Employees) │
│ Port 9-16: VLAN 20 (Guests) │
│ Port 17-24: VLAN 30 (Servers) │
└──────────────────────────────────────┘