With NAT, hosts are given addresses in one of these ranges, and these addresses are translated before going out onto the Internet Generally, the host at the border figures out where the internal host was trying to reach, and contacts that destination on behalf of the internal host. Any responses are passed on to the internal host by the border router. Some applications may have problems, because they embed IP addresses in their data. We also won't get into the security issues mentioned in the text. This is an older configuration.
Details are in the book.
The book spends a bit of time discussing OS specifics related to networking. What will probably stand out is how different the configuration of networking is on each platform. It was actually much easier in the old days when all you had to do was edit the appropriate script. You should spend some time learning the specifics of how networking works on whatever platform you're using. And know that it's likely to be different on other flavors of Unix.