As the number of networks that were connected to the ARPAnet increased, communication among the computers became a problem. Common standards were required for communication because the hardware and the software that were used were vendor-specific. A common protocol was necessary for communication between the computers. This led to the creation of TCP and IP. With the increase in the number of requirements, several protocols were created to address all the requirements. This also led to the creation of a new reference model, called the TCP/IP reference model. The TCP/IP reference model consists of four layers: Application, Transport, Internet, and Network Interface, as shown in Figure 1.5.