Sharing one IP address among multiple Apple devices
Yes, but with caveats.

MacOS X has an "internet sharing" mode in the "Sharing" control panel, but you have to use different network interfaces for the "internet facing" and "internal facing" sides of your network.

So if you're connected to the Internet over your ethernet port, you can share your connection to WiFi clients.  What you can't do is connect and share over the same port.