Cisco CCNA Preparation · lesson 4 of 36 · 25 min
Ethernet — The Dominant LAN Technology
Ethernet (IEEE 802.3) is the dominant wired LAN technology. Originally developed by Xerox, DEC, and Intel in the 1970s, it has evolved from 10 Mbps coaxial cable to 400 Gbps fibre. Understanding Ethernet fundamentals is essential for the CCNA — it underpins virtually every enterprise network.
CSMA/CD — Legacy Media Access
CSMA/CD (Carrier Sense Multiple Access with Collision Detection) was the original Ethernet media access method for shared (half-duplex) networks:
Carrier Sense: Before transmitting, a device listens to check if the medium is idle. Multiple Access: Multiple devices share the same medium. Collision Detection: If two devices transmit simultaneously, a collision occurs. Both stop, wait a random backoff time (binary exponential backoff), then retry.
CSMA/CD is largely obsolete in modern networks because full-duplex switched Ethernet eliminates collisions entirely — each device has a dedicated point-to-point link to the switch.
Exam tip
CSMA/CD is still tested on the CCNA. Know that it applies to half-duplex shared media. Modern full-duplex switched networks do not use CSMA/CD. Hubs create collision domains; switches eliminate them.
MAC Addresses
A MAC (Media Access Control) address is a 48-bit (6-byte) hardware address burned into a NIC by the manufacturer. Written as 12 hex digits: 00:1A:2B:3C:4D:5E or 001A.2B3C.4D5E (Cisco notation).
OUI (Organizationally Unique Identifier): First 3 bytes (24 bits) identify the manufacturer. Last 3 bytes are the device-specific identifier assigned by the manufacturer.
Special MAC addresses: FF:FF:FF:FF:FF:FF = broadcast (sent to all devices on the segment). 01:00:5E:xx:xx:xx = IPv4 multicast. 33:33:xx:xx:xx:xx = IPv6 multicast.
Ethernet Frame Format
MTU
The MTU (Maximum Transmission Unit) for standard Ethernet is 1500 bytes (the maximum payload size). Jumbo frames extend this to 9000 bytes on supported hardware, reducing overhead for large transfers.
Duplex and Speed
Key Terms
Key Terms
Knowledge Check — 10 Questions
Ethernet Fundamentals
What does CSMA/CD stand for?
- Carrier Sense Multiple Access with Collision Detection
- Channel Sharing Multiple Access with Collision Delay
- Carrier Signal Media Access with Collision Delay
- Channel Sense Multiple Access with Collision Detection
Answer: Carrier Sense Multiple Access with Collision Detection
CSMA/CD: devices listen before transmitting (Carrier Sense), share the medium (Multiple Access), and detect and handle collisions (Collision Detection).
In modern full-duplex switched Ethernet, why is CSMA/CD not needed?
- Switches are too fast for collisions
- Each device has a dedicated point-to-point link to the switch — no shared medium
- Full-duplex uses a different protocol
- Switches use CSMA/CA instead
Answer: Each device has a dedicated point-to-point link to the switch — no shared medium
Full-duplex switched Ethernet gives each device its own collision domain (a single link to the switch). With no shared medium, collisions cannot occur.
How many bytes is a MAC address?
- 4
- 6
- 8
- 16
Answer: 6
MAC addresses are 48 bits = 6 bytes, written as 12 hex digits (e.g. 00:1A:2B:3C:4D:5E).
What does the OUI portion of a MAC address identify?
- The device model
- The manufacturer
- The network segment
- The IP address mapping
Answer: The manufacturer
The OUI (first 3 bytes of a MAC address) is assigned by IEEE to each manufacturer. It identifies who made the NIC.
What is the maximum payload size of a standard Ethernet frame?
- 512 bytes
- 1024 bytes
- 1500 bytes
- 9000 bytes
Answer: 1500 bytes
Standard Ethernet MTU is 1500 bytes. Jumbo frames extend this to 9000 bytes on supported hardware.
What EtherType value indicates an IPv4 packet in the Ethernet frame?
- 0x0806
- 0x86DD
- 0x0800
- 0x8100
Answer: 0x0800
0x0800 = IPv4. 0x0806 = ARP. 0x86DD = IPv6. 0x8100 = 802.1Q VLAN tag.
What is the broadcast MAC address?
- 00:00:00:00:00:00
- FF:FF:FF:FF:FF:FF
- 01:00:5E:00:00:01
- 33:33:00:00:00:01
Answer: FF:FF:FF:FF:FF:FF
FF:FF:FF:FF:FF:FF is the Ethernet broadcast address. Frames sent to this address are delivered to all devices on the local segment.
What is the purpose of the FCS field in an Ethernet frame?
- Identifies the upper-layer protocol
- Provides error detection using CRC
- Synchronises clocks between sender and receiver
- Identifies the VLAN
Answer: Provides error detection using CRC
The FCS (Frame Check Sequence) contains a CRC value. The receiver recalculates the CRC and compares — if they differ, the frame is discarded.
The minimum Ethernet frame payload is 46 bytes. If the data is smaller, it is padded to meet this minimum. The maximum payload is _____ bytes.
Answer: 1500
Standard Ethernet MTU is 1500 bytes. The minimum payload is 46 bytes (padded if necessary) to ensure the frame is long enough for collision detection.
Which Ethernet standard uses Cat5e or Cat6 UTP cable and operates at 1 Gbps?
- 100BASE-TX
- 1000BASE-T
- 10GBASE-T
- 1000BASE-SX
Answer: 1000BASE-T
1000BASE-T (Gigabit Ethernet) uses Cat5e or Cat6 UTP cable at 1 Gbps over a maximum distance of 100 metres.
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