This figure depicts the path of a message from a client to a server and the server's reply. For example, if this were a Web application, the client might send a get-page message to the server and the server would reply with either the page or an error message.
Note that the application programs are unaware of the nature of the network. The client application program sends information to the server application program, which sends information back to the client.
Neither program is aware of the details of delivery of the message -- how large the packets were, how long they took to arrive, whether they arrived in or out of order, whether any error retransmission was necessary, whether the connections between machines were wired or wireless or fast or slow, etc.
In this case, the client and server are both on the same network since information flows directly between their LAN interfaces.