Preamble (7 bytes)

• The preamble in an Ethernet frame is a sequence of bits used to signal the start of the frame and synchronize communication between the sender and receiver

SFD (1 byte)

• The SFD or Start Frame Delimiter is a special byte that marks the end of the preamble and signals the start of the actual data portion of the Ethernet frame

Source MAC Address (6 byte)

• The 48 bit MAC address that helps the receiving device identify the sender 

Destination MAC Address (6 byte)

• The 48 bit MAC address of the device that is intended to receive the frame

Type (2 byte)

• The Type field that indicates the protocol or the type of data encapsulated in the frame's payload such as IPv4/IPv6, ARP, etc

Data (46 - 1500 bytes)

• The Data field contains the actual data being transmitted between devices
• The maximum size of the data field is determined by the MTU which is usually 1500 bytes for Ethernet

Padding

• The Padding field is used if the payload or data field of the frame is smaller than 46 bytes. Padding is added to meet the minimum size requirement for an Ethernet frame (64 bytes total)

CRC (4 bytes)

• The purpose of the CRC or Cyclic Redundancy Check is to ensure that the data transmitted in the frame has not been corrupted during transmission

Version (4-bit)

• The version field indicates the protocol version of the packet being transmitted: IPv4/IPv6

Internet Header Length (4-bit)

• The IHL field indicates the length of the IP header in 32-bit words (4-byte units) 

Type of Service (8-bit)

• The ToS field specifies how the packet should be treated in terms of its priority and handling during transmission across the network
• This field is intended for QoS or Quality of Service helping layer 3 devices make decisions about how to prioritize traffic

Total Length (16-bit)

• The total length field specifies the total length of the IP packet including both the header and data varying from 0 - 65,535 bytes

Identification (16-bit)

• The identification field is used to uniquely identify each packet or fragment of a packet in a network 
• When an IP packet is too large to be transmitted in a single unit, it is fragmented into smaller packets
• The Identification field helps to link those fragments together so they can be reassembled correctly by the receiver

Flags (3-bit)

• The flags field is used to control the fragmentation of the IP packet and determines whether a packet is fragmented, whether more fragments are expected, and how the packet should be handled during fragmentation 

Fragment Offset (13-bit)

• The fragment offset field is used to indicate the position of a fragment in the original unfragmented packet and plays a critical role in reassembling fragmented packets once they reach their destination 

Time to Live (8-bit)

• The TTL field plays an important role in controlling the lifetime of a packet in a network 
• The TTL value helps prevent packets from circulating indefinitely if there is a routing loop or other issues in the network 
• The TTL value specifies the number of hops the packets can pass through before being discarded

Protocol (8-bit)

• The protocol field indicates what type of higher layer protocol is being carried in the packet's payload 
• The protocol field helps the receiving device understand how to interpret the data, directing it to the appropriate protocol handler
• Example protocols being (e.g., TCP, UDP, ICMP, OSPF)

Header Checksum (16-bit)

• The header checksum field is used for error detection to detect any changes or corruption that might occur in the header during transmission through the network
• When the packet reaches its destination or any other intermediate router, the receiver recalculates the checksum for the header and compares it with the value in the Header Checksum field

Source Address (32-bit)

• The 4byte address that specifies the IP address of the sender or source of the packet

Destination Address (32-bit)

• The 4byte address that specifies the IP address of the intended target or destination of the packet

Options (0 - 40 bytes)

• The options field provides a way for certain features or specific instructions to be added to the packet, such as routing controls or timestamps
• This field is typically used in special cases and is not always present in most data packets 

Data (variable length)

• The data field refers to the part of the packet that carries the payload, the actual data being transmitted from the source to the destination