Network packet

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

A packet is a network-layer protocol data unit consisting of a segment combined with a Layer 3 header.

A network packet is a Layer 3 protocol data unit (PDU): a Layer 3 header combined with a Layer 4 segment. Its encapsulated segment is the packet’s payload. Packets matter because Layer 3 delivers segments to the correct destination host, while Layer 2 carries each packet to the next hop. section 4.3.2 section 4.4

Place the packet in the encapsulation sequence:

• Application data plus a Layer 4 header forms a segment, or L4PDU. section 4.3.2
• A Layer 3 header added to the segment forms a packet, or L3PDU. section 4.3.2
• A Layer 2 header and trailer added to the packet form a frame, or L2PDU. section 4.4
• At the destination, the host removes the frame’s Layer 2 information, then the packet’s Layer 3 header, then the segment’s Layer 4 header. section 4.3.2

PC1 sends a packet to PC3

PC1 creates a packet addressed to PC3 and uses ARP to learn the default gateway’s MAC address. The packet travels inside a frame to the gateway. Each router selects the best route and forwards the packet in a new frame addressed to the next hop. The Layer 3 destination remains PC3 throughout the journey, while the Layer 2 frame addresses change at every hop. section 10.0 section 10.1

Figure 4.7 Application data encapsulated in a Layer 4 header is a segment (L4PDU); a segment encapsulated in a Layer 3 header is a packet (L3PDU); and a packet encapsulated in a Layer 2 header/trailer is a frame (L2PDU). The encapsulated contents of each PDU are that PDU’s payload.

Trace the relationship among application data, segments, packets, frames, and payloads. section 4.3.2

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes

Defines packet structure and follows forwarding across hops, while stopping short of detailed header-field analysis.

Examples

Uses PC1-to-PC3 delivery and the return path to show changing frames, routing decisions, ARP, and learned tables.

What it assumes

Builds from the TCP/IP model and assumes familiarity with Layer 2 frames, Layer 4 segments, ARP, switching, and routing.

What you can do after

Evaluate/understand how packets become frames, cross routed networks, and are de-encapsulated at the destination.

How it connects

Connects packets with segments, frames, encapsulation, de-encapsulation, ARP, switching, routing, MAC addresses, and IP addresses.

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Related concepts

• IPv6used by · IPv6 is the sixth version of the Internet Protocol, using 128-bit network addresses for packet-based communication.
• protocol data unitkind of · A protocol data unit is a unit of data consisting of a protocol payload together with the control information added at a networking layer.
• framepart of · An Ethernet frame is a link-layer protocol data unit that carries a network-layer packet and associated framing information across an Ethernet link.

This explainer was generated by Manning's knowledge graph from the text of Acing the CCNA Exam: Volume 1 Fundamentals and Protocols. The section links point to the parts of the book it draws on.