Getting Internet Sharing to Work on Mountain Lion

My wife is over in Denmark by herself at the moment but wanted to share her in room internet connection (a cabled Ethernet connection) to her other devices over Wi-Fi. She had recently upgraded her MacBook Air to Mountain Lion (Mac OS X 10.8) but had used internet sharing on that computer in the past on earlier versions of the operating system.

Anyway, attempting to setup internet sharing proved a little challenging and presented issues. Normally, you should just be able to go via the Sharing applet in System Preferences and choose the Internet-facing connection and to which other network connection you want to share. In this case, we wanted to share from the Thunderbolt Ethernet adapter to the Wi-Fi adapter. Unfortunately, it’s not enough to simply do that it seems after many failed attempts to get it to work – you actually need to hard code an IP address for the Wi-Fi interface on the Mac doing the sharing (at least that is what worked).

This is what we did:

1. Undo all network sharing,
2. Go into System Preferences and then Network,
3. Click on Thunderbolt Ethernet (or whichever network interface you wish to share from) then click the Advanced button,
4. Click the TCP/IP tab and take note of the Router address,
5. Click the DNS tab and take note of the DNS addresses and then click OK,
6. Click on Wi-Fi (or whichever network interface you wish to share to) then click the Advanced button,
7. Click the TCP/IP tab up the top,
8. Next to Configure IPv4 select Manually,
9. Next to IPv4 Address type in 10.0.0.1, 172.16.1.1 or 192.168.1.1 (so long as it doesn’t match the first number in the router address of which you took note in Step 3),
10. Next to Subnet Mask type in 255.255.255.0
11. Next to Router type in the address noted in Step 4,
12. Click the DNS tab,
13. Type in the DNS addresses noted in Step 5 (just click the plus symbol underneath when you want to add a new one),
14. Click OK then Apply,
15. Go into System Preferences and then Sharing,
16. Click on Internet Sharing in the list on the left,
17. Next to Share your connection from choose your connection from which to share (in my case, Thunderbolt Ethernet),
18. Next to To computers using, tick the network interface to which you want to share the internet connection (in my case, Wi-Fi),
19. If you are sharing to Wi-Fi then click the Wi-Fi Options button (otherwise skip down to Step 22),
20. Give your network a suitable name, select appropriate security (WPA2 is available otherwise WEP) and provide a password,
21. Click OK,
22. If you are prompted to start Internet Sharing accept the offer otherwise tick the box next to Internet Sharing on the left.

Once done, you should be able to hook up your other devices and get a connection to the web and this certainly got things working on the other side of the world after a lengthy LogMeIn and Skype session!