7.21 I’ve heard that there are special IP addresses reserved for private networks and that it is a good idea to use addresses in this range internally. What are these addresses?

These are the ranges of addresses reserved for private networks:

10.0.0.0 - 10.255.255.255

172.16.0.0 - 172.31.255.255

192.168.0.0 - 192.168.255.255

Packets for these addresses are almost guaranteed to stay within your network. Because the addresses are reserved for private use, routers in public networks are configured to drop packets that use them either as source or destination. That means that you won't have to re-address your network all at once if you eventually connect to the Internet.