Think of an IP packet like a letter¶

A packet can be pictured as a letter moving through a postal system. It is a teaching analogy, not a literal description.

Postal idea	Network idea
Envelope	Packet as a whole
Address on the envelope	IP header addressing
Paper inside	Payload
Sorting offices	Routers
Delivery network	Interconnected networks

Routers use header information to decide where to forward traffic without needing to understand the content. A destination address helps delivery; a source address helps return traffic, but neither proves who wrote the message.

A large photograph or webpage normally becomes many packets. Packets can be lost, copied, retransmitted, reordered or sent through changing routes - behaviour with no exact postal equivalent. NAT, VPNs and proxies can also change which source address is visible.

The analogy is most useful for separating header from payload and delivery from authorship. For the technical detail, see the IP packet card.