TCP/IP packet.

Ideally, one working on computer network related field never has to get into the nitty gritty of data packets.  Unfortunately, he has to delve into it sooner or later usually for troubleshooting.

Fortunately, there are plenty of technical notes providing excellent reference examing data packets. It is rarely needed to deal with any layer below layer 3 Network Layer of the Open Systems Interconnection model. TCP/IP are by far the most frequently used layer 4/layer 3 (i.e. Transport Layer/Network Layer).

IP Packet Structure (Network Layer, or layer 3)

TCP Header Format (Transport Layer, or layer 4)

HTTP (Application Layer, or layer 7)