In CS35110 lecture one today, we briefly looked at the network stack. Particularly this diagram:
At each level the protocol module at that level communicates only with the layer above and the layer below. Thus the network layer may received data from the transport layer that it must enclose in a network packet (like an envelope) before passing that packet down to the link layer.
the protocol layers are thus stacked on top of each other in the protocol suite.
Bear in mind that packets are really just sequences of data, and all that is really transmitted over the physical medium (the wires or wireless medium of the internet) are long strings of binary data.
Next lecture we will look particularly at issues such as why packets are used, and we will look at the network layer in more detail. However, if there are specific issues you want to raise, add comments here and we can discuss those issues in the next lecture.