Figure 1 shows the network address and the data link address contained in a message.  The physical layer contains the timing and synchronization bits.  The data link layer contains the destination and source physical addresses.  The network layer contains the destination and source logical network addresses.  The transport layer contains the destination and source port numbers and the upper layers contain the encoded application data. Figure 2 shows an IP packet traversing several routers. The source and destination IP address are inserted into the packet header.