CCNA Discovery 1, Chapter 5

5.2.2: As mentioned on Day 20, you can assign a private addressing scheme to an internal network; however, these addresses cannot be routed over the Internet. Table 18-1 displays the reserved addresses for private networks.

Table 18-1. RFC 1918 Private Networks

If you create a private network with one of these address ranges, your internal hosts can still connect to the Internet through a router configured for NAT. A router can receive a public Internet-routable address from the ISP and provide Internet connectivity for the hosts on the local private network. The router uses NAT to exchange private IP addresses for a public IP address or a pool of public IP addresses. This change (or translation) of the address allows an internal host to appear as though it has a public IP address for the purpose of Internet communication. In addition, NAT can provide security, because an outside computer on the Internet cannot discover the private IP address of a host on the internal network.