What is Dealing with Incompatibility ?

 The incompatibility issues are handled at two levels:

i) Hardware Issues
At the hardware level, an additional component called router is used to connect
physically distinct networks as shown in Figure 1. A router connects to the network
in the same way as any other computer. Any computer connected to the network has a
Network Interface Card (NIC), which has the address (network id+host id), hard
coded into it. A router is a device with more than one NICs. Router can connect
incompatible networks as it has the necessary hardware (NIC) and protocols
(TCP/IP).

ii) Software Issues
The routers must agree about the way information would be transmitted to the
destination computer on a different network, since the information is likely to travel
through different routers, there must be a predefined standard to which routers must
confirm. Packet formats and addressing mechanism used by the networks may differ.
One approach could be to perform conversion and reconversion corresponding to
different networks. But this approach is difficult and cumbersome. Therefore, the
Internet communication follows one protocol, the TCP/IP protocol suite. The basic
idea is that it defines a packet size, routing algorithms, error control, flow control
methods universally.

TCP/IP Protocols It would be unwise to club all these features in a single piece of software ─ it would
make it very bulky. Therefore, all these features are logically sub-grouped and then
the sub-groups are further grouped into groups called layers. Each layer has an
interface with the adjacent layers, and performs specific functions.

Need for Layering
Since it is difficult to deal with complex set of rules, and functions required for
computer networking, these rules and functions are divided with logical groups called
layers. Each layer can be implemented interdependently with an interface to other
layers providing with services to it or taking its services like flow control and error
control functions are grouped together and the layer is called data link layer. Speech
in telephone conversation is translated, with electrical segments and vice-versa.
Similarly in computer system the data or pattern are converted into signals before
transmitting and receiving. These function and rules are grouped together in a layer
called physical layer.