Internet Sharing on Your Mac with Mountain Lion
If your Mac (running OS X Mountain Lion) has an Internet connection and another Mac nearby doesn’t, you can enable Internet Sharing, and the other Mac can share your Internet connection. The following steps show you how:
Open the Sharing System Preferences pane by launching the System Preferences application (from the Applications folder, Apple menu, Launchpad, or Dock) and clicking the Sharing icon.
Select the Internet Sharing check box in the list of services on the left.
Choose the connection you want to share — AirPort, FireWire, or Ethernet — from the Share Your Connection From pop-up menu.
Select the check boxes next to connections other computers will use — Wi-Fi, Ethernet, or Built-In FireWire.
The following figure shows Internet Sharing configured to share an Ethernet Internet connection with another Mac by using Wi-Fi.
(Optional) Click the Wi-Fi Options button to name, select a wireless channel for, enable encryption for, and/or set a password for your shared network.
That’s all there is to it.