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Data EncapsulationDomain 1.0 / 3.0
Packet Layer 3 Protocol Data Unit (PDU)
The Protocol Data Unit utilized at Layer 3 (the Network Layer) of the OSI model. While Layer 2 Frames exist only to move data across a single local link, the IP Packet contains the logical addressing necessary to route data end-to-end across the entire global internet.
Anatomy of an IPv4 Packet Header
| Header Field | Purpose |
|---|---|
| Source IP | 32 Bits: The logical IP address of the original sending host. |
| Destination IP | 32 Bits: The logical IP address of the ultimate destination host. Routers use this field to perform Longest Prefix Matches against their routing tables. |
| TTL (Time to Live) | 8 Bits: A loop-prevention mechanism. Starts at a set number (e.g., 64 or 255) and is decremented by 1 every time the packet passes through a router. If it hits 0, the router destroys the packet. |
| Protocol | 8 Bits: Identifies the Layer 4 payload encapsulated inside. (e.g., 1 for ICMP, 6 for TCP, 17 for UDP). |
| DSCP | 6 Bits: Differentiated Services Code Point. Used for Layer 3 Quality of Service (QoS) to prioritize critical traffic like VoIP over bulk data transfers. |
CCNA Exam Gotchas
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End-to-End Integrity
A massive conceptual point for the CCNA: As data travels across the internet, the Layer 2 Frame (Source/Dest MAC) is stripped off and rewritten by every single router along the path. However, the Layer 3 Packet (Source/Dest IP) remains completely unchanged from the original sender to the final receiver (unless it passes through a NAT boundary).