How the Protocol Suite works
Colin Burnett's diagram below illustrates how applications use lower layer protocols to transmit messages across the Internet. Notice how routers use the network layer to route messages - network layer packets have the destination IP address.
When a message is passed down a layer it is encapsulated within a package appropriate to that layer.
When a message is passed up a layer the packaging for that layer is removed to leave a message appropriate to that layer.
Next: TCP/IP Activities