Extensibility
It is fair to say that the uses to which the Internet is put today were not foremost in the minds of the designers of IPv4. The open and simple nature of IPv4's design means that it has been possible to build applications such as remote-login, the Web and peer-to-peer file sharing without repeatedly returning to the drawing board. We mentioned before the notion of networking "layers," originating with the OSI 7-layer model described in Chapter 1, which is partially responsible for this, but it's a testimony to good engineering that there are so few hidden dependencies between levels. IPv6 also preserves this decoupling.
The simplicity of IPv4, discussed in Section 2.1 of this chapter, is almost certainly another reason why it has been extended and pushed in unusual directions. It works on the principle of giving people simple building blocks, which can be assembled in weird and wonderful ways.
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