How does NAT affect public and private IPv4 address usage?

• A. Public addresses are routable on the Internet.
• B. Private addresses are not routable on the Internet; NAT translates them to a public address for external communication.
• C. NAT translates public addresses to private addresses for internal use.
• D. Private addresses are not routable on the Internet; NAT translates them to a private address for internal use.

Answer: (B)

NAT lets devices inside a private network use private IPv4 addresses while the Internet relies on public addresses that are routable globally. When a device with a private address sends traffic outside, the router translates that private source address (and often port) into a public IP address that the Internet can reach. The router keeps a table of these translations so that replies coming back from the Internet can be mapped back to the correct private device inside the network. This is what enables multiple private addresses to share a single (or small set of) public addresses and still communicate externally.

In practice, private addresses aren’t reachable directly from the Internet, but through NAT they appear as a public address, allowing outbound connections and return traffic to be correctly routed. If you need inbound connections from the Internet to a specific internal device, you’d set up port forwarding or a similar NAT rule to direct that traffic to the appropriate private host.