A running computer always knows its own IP address and Ethernet address. If a computer is connected to 2 separate Ethernets it is as in Figure 3. Ethernet Cable 1 Figure 3. It performs this multiplexing in either direction to accommodate incoming and outgoing data. An IP module with more than 1 network interface is more complex than our original example in that it can forward data onto the next network.
Data can arrive on any network interface and be sent out on any other.
As you can see from the figure, the forwarded IP packet never touches the TCP and UDP modules on the IP-router. Some IP-router implementations do not have a TCP or UDP module. The IP module is central to the success of internet technology. The IP header contains the IP address, which builds a single logical network from multiple physical networks. 2.6 Physical Network Independence IP hides the underlying network hardware from the network applications.