The Ethernet II Frame

The Ethernet II frame: Preamble SFD Dest MAC Src MAC Type/Length Data FCS 7 B 1 B 6 B 6 B 2 B 46–1500 B 4 B Preamble — synchronises the NIC with the incoming bit stream. Start Frame Delimiter — signals the frame actually begins here. Destination / Source MAC — self-explanatory. Length or […]

The IPv6 Header

A fixed 40-byte Layer 3 header (RFC 8200) designed for fast hardware forwarding — no checksum, no options inside the base header, extensions as modular headers. Fields Field Size Purpose Version 4 bits Always 6 Traffic Class 8 bits QoS (DSCP + ECN) Flow Label 20 bits Flow identity for QoS/load-balancing Payload Length 16 bits […]

The IPv4 Header

The IPv4 header is a variable Layer 3 header (RFC 791, 20–60 bytes) containing everything routers use to forward, fragment and deliver packets. Fields (exam critical) Field Size Purpose Version 4 bits Always 4 IHL 4 bits Header length in 32-bit words (5–15) DSCP / ECN 8 bits QoS prioritisation + congestion signal Total Length […]