The Data Link Layer

Unicast, Multicast, and Broadcast

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Unicast

A unicast transmission is always meant for just one receiving address. If the least significant bit in the first octet of a destination address is set to zero, it means that ethernet frame is intended for only the destination address.

Multicast

If the least significant bit in the first octet of a destination address is set to one, it means you're dealing with multicast frame.

Broadcast

Broadcast Address : FF:FF:FF:FF:FF:FF

Address Resolution Protocol

Dissecting an Ethernet Frame

Data Packet

An all-encompassing term that represents any single set of binary data being sent across a network link

Ethernet Frame

A highly structured collection of information presented in a specific order

Preamble : 8 bytes

Destination Address : 6 bytes

Source Address : 6 bytes

Tag (VLAN header) : 4 bytes

Ether-type : 2 bytes

Payload : 0-1,500 bytes

FCS : 4 bytes

Preamble

8 bytes (or 64 bits) long, and can itsepf be split into two sections

First 7 bytes works partially as buffer between frames and used by network interfaces to synchronize internal clockes they use to regulate the speed at which they send data.

Last byte is called SFD (Start Frame Delimiter)

Start Frame Delimiter (SFD)

Signals to a receiving device that the preamble is over and that the actual frame contents will now follow

Destination MAC Address

The hardware address of the intended recipient

<-> Source MAC Address (where frame originated from)

MAC Address is 48 bits, 6 bytes, long.

EtherType Field

16 bits long and used to describe the protocol of the contents of the frame

VLAN header (Tag)

Indicates that the frame itself is what's called a VLAN frame. If a VLAN header is present, the EtherType field follows it.

Virtual LAN (VLAN)

A technique that lets you have multiple logical LANs operating on the same physical equipment

Any frame with the VLAN tag will only be delivered out of Switch interface configured to relay that specific tag.

This way, you can have single physical network that operates like multiple LANs.

VLANs are often used to segregate different forms of traffic.

Payload

In network terms, is the actual data being transported, which is everything that isn't a header

Contains all of the data from higher layers such as IP, Transport, Application Layers that are actually being transmitted

Frame Check Sequence

A 4-byte (or 32-bit) number that represents a checksum value for the entire frame

This checksum value is calculated by performing what's known as a cyclical redundancy check against the frame.

Cyclical Redundancy Check (CRC)

An important concept for data integrity, and is used all over computing, not just network transmissions

Ethernet itself only reports on data integrity, it doesn't perform not data recovery.