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The OSI Model
I instruct my browser to fetch a web page. It sends an HTTP request down the stack.
TCP breaks it up in segments of optimal size, assigns sequence numbers, and passes the segments down to IP.
IP creates packets, assigns source and destination addresses, and passes them down the stack.
Ethernet frames are created to carry my data to the next node.
My network card translates these frames to electricity and puts them on the wire.
Notes:
Each layer of the OSI model knows nothing about what's happening over or below it. Layers have clearly defines interfaces, through which they pass data to their neighbors
This is the advantage of a layered approach – changes in the implementation of the Transport Layer have no effect on the Network Layer – thus IP doesn't care whether it's carrying TCP or UDP or SUPERDUPER data – it just encapsulates whatever chunks of data are passed to it, and sends them to the lower layers.