THE INTERNET LAYER PROTOCOLS

In the DoD model, there are two main reasons for the Internet layer’s existence: routing and providing a single network interface to the upper layers. None of the other upper- or lower-layer protocols have any functions relating to routing that complex and important task belongs entirely to the Internet layer.

 The Internet layer’s second duty is to provide a single network interface to the upper-layer protocols. Without this layer, application programmers would need to write “hooks” into every one of their applications for each different Network Access protocol. This would not only be a pain in the neck, but it would lead to different versions of each application—one for Ethernet, another one for Token Ring, and so on. To prevent this, IP provides one single network interface for the upperlayer protocols. That accomplished, it’s then the job of IP and the various Network Access protocols to get along and work together.

All network roads don’t lead to Rome—they lead to IP. And all the other protocols at this layer, as well as all those at the upper layers, use it. Never forget that. All paths through the DoD model go through IP.
The following list describes the protocols at the Internet layer:

• Internet Protocol (IP)
• Internet Control Message Protocol (ICMP)
• Address Resolution Protocol (ARP)
• Reverse Address Resolution Protocol (RARP)
• Proxy ARP