Dissecting an Ethernet Frame 🕸️
To wrap up, we'll round out your understanding of the basics of networking by dissecting an Ethernet frame. Understanding the networking basics is the first step in building a really strong foundation of networking knowledge that you'll need in IT support.
📦 Data Packets and Ethernet Frames 📦
A data packet is an all-encompassing term that represents any single set of binary data being sent across a network link. The term data packet isn't tied to any specific layer or technology; it just represents a concept: one set of data being sent from point A to point B.
Data packets at the Ethernet level are known as Ethernet frames. An Ethernet frame is a highly structured collection of information presented in a specific order. This way, network interfaces at the physical layer can convert a stream of bits traveling across a link into meaningful data or vice versa.
📜 Anatomy of an Ethernet Frame 📜
An Ethernet frame consists of several mandatory sections, each with a fixed size. Let's break down the different parts:
Preamble: The preamble is the first part of an Ethernet frame. It is eight bytes or 64 bits long and can be divided into two sections:
The first seven bytes are a series of alternating ones and zeros. These act partially as a buffer between frames and can also be used by the network interfaces to synchronize internal clocks they use to regulate the speed at which they send data.
The last byte in the preamble is known as the Start Frame Delimiter (SFD). This signals to a receiving device that the preamble is over and that the actual frame contents will now follow.
Destination MAC Address: Immediately following the start frame delimiter, comes the destination MAC address. This is the hardware address of the intended recipient.
Source MAC Address: The source MAC address follows the destination MAC address, indicating where the frame originated from. Each MAC address is 48 bits or six bytes long.
Ether-type Field: The Ether-type field is 16 bits long and used to describe the protocol of the contents of the frame. We'll be doing a deep dive on these protocols later.
VLAN Header (Optional): Instead of the Ether-type field, you can also find what's known as a VLAN header. It indicates that the frame itself is a VLAN frame. VLAN stands for virtual LAN and allows multiple logical LANs to operate on the same physical equipment. VLANs are usually used to segregate different forms of traffic.
Data Payload: After the VLAN header or Ether-type field, you'll find the data payload of an Ethernet frame. The payload is the actual data being transported and includes all the data from higher layers, such as the IP, transport, and application layers. The data payload of a traditional Ethernet frame can be anywhere from 46 to 1500 bytes long.
Frame Check Sequence (FCS): Following the data payload, there's the frame check sequence. It is a four-byte or 32-bit number that represents a checksum value for the entire frame. The FCS is calculated using a cyclic redundancy check (CRC), which is a mathematical transformation that ensures data integrity during transmission.
✅ CRC and Data Integrity ✅
A cyclic redundancy check (CRC) is an important concept for data integrity and is used all over computing, not just in network transmissions. It is a mathematical transformation that uses polynomial division to create a number representing a larger set of data. When performing a CRC against a set of data, you should end up with the same checksum number.
In the context of an Ethernet frame, the frame check sequence (FCS) serves as a CRC to validate the integrity of the received data. If the computed checksum at the receiving end doesn't match the checksum in the FCS field, the data is discarded due to potential loss or corruption during transmission. Ethernet itself focuses on data integrity and relies on higher-layer protocols to handle data recovery and retransmission.
Congratulations! You now have a solid grasp of networking basics. Great job! 🎉
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