Ethernet, MAC addresses and frames

The Ethernet II frame field by field, the 48-bit MAC address and its three kinds (unicast, multicast, broadcast), error checking, CSMA/CD and duplex.

The outline, topic by topic

Each topic is weighted the way the exam weighs it.

Ethernet II frame: what is it?
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The unit of data on an Ethernet link: destination address, source address, EtherType, 46 to 1,500 bytes of data, then a 4-byte error check.
The first three fields make 14 bytes (6 + 6 + 2) and the error check 4: a complete frame is therefore 64 to 1,518 bytes long.
A frame carries the EtherType 0x0806. What does its data hold?
- An ARP message
- An IPv4 packet, carried under 0x0806
- An IPv6 packet, carried under 0x0806
- A TCP segment sent straight over Ethernet
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An ARP message
0x0800 means IPv4, 0x86DD IPv6 and 0x0806 ARP. A TCP segment is never placed straight into a frame: it is first wrapped in an IP packet.
A MAC address is … bits long, that is six bytes.
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A MAC address is 48 bits long, that is six bytes.
Six bytes of two hexadecimal digits: 3c:22:fb:10:9a:01. The first three name the manufacturer, the last three the card.
MAC address: what is it?
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The 48-bit identifier of a network interface, written as six hexadecimal bytes separated by colons, whose first three bytes name the manufacturer.
It only matters on the local network: a router rewrites the frame's MAC addresses at every hop. An interface can also get a locally administered address, with no manufacturer, such as 02:42:ac:11:00:02 on a Docker container.