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, or (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
  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!


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    • Rico on November 2, 2012 at 06:05
    • Reply

    If i were to share to an ethernet connection, do you know what i would have to put as DNS, Mask, and IP on the computer that is connected to me and sharing my internet connection?

    1. Normally, you wouldn’t need to hardcode any of that information on the computer sharing the internet connection nor on the client computers which are connecting to the sharing computer. In this situation the computer doing the internet sharing was causing issues requiring me to hard code the information there but I didn’t have to enter anything in on the client devices.

      However, if you do need hard code your client devices as well you would enter in the following:

      IP Address: increment the last number of the router address by one (so if you had for the router put in for the first client device, for the next, etc),
      Subnet mask: this needs to match what you entered in on the internet sharing computer (so in the case of this article),
      DNS: the same numbers used on your public facing internet connection or you could use something like Google’s DNS of and instead.

      Hope that answers your question 🙂

      On computers sharing the connection from your computer you shouldn’t need to set that information as it should

    • Joe on November 22, 2012 at 13:57
    • Reply

    This didn’t work. Trying to share from imac ethernet connected to ipad wifi connected. Just upgraded to Mountain Lion. When I was on leopard there was a little arrow in the wifi icon up top. Now there is some kind of a box/bus square icon in greyed out pie. The ipad sees my network. I connect. It checks the network. Everything is go. Click on safari: you are not connected to internet. Help?

    • W on December 29, 2012 at 22:04
    • Reply

    Awesome directions! This worked perfectly on my MacBook running Mountain Lion 10.8.2. Thanks a ton for posting this!

    1. Very glad to hear! 🙂

    • Lars on February 2, 2013 at 20:56
    • Reply

    I’m trying to share my internet from WI-FI to ethernet on my iMac. The computer that’s connected to ethernet got IP through DHCP, and can resolve DNS, but gets no response. I have no idea what the problem is. My firewall is off, and I’m not running any server software on my iMac.

    1. Interesting. Are the two networks on different subnets or are the same or overlap?

    • Chelsea on February 7, 2013 at 13:17
    • Reply

    Thank you!!! you are a lifesaver!!!

    1. No worries at all 🙂

    • Lanz on February 18, 2013 at 04:41
    • Reply

    This is so so helpful. Thank you for sharing. I got one thing that I’m really confused. My friend’s laptop PC cannot connect to my home wifi for some reason but he can connect to my laptop PC using ad-hoc and share internet. My laptop is connected to internet via wifi. Can I do this in my Mac running Mountain Lion? I am connected to internet via wifi and want to share internet to my friends laptop PC via wifi. Thank you in advanced.

    1. Hi, Lanz,

      I don’t believe any version of Mac OS X allows “Wi-Fi virtualisation” which allows you to be connected (or host) more than one Wi-Fi network simultaneously. In contract, Windows 7 and 8 do support this functionality so long as the Wi-Fi hardware also supports it but you may not want to go to the trouble of installing Windows on your Mac just to share internet access.

      I think it would be far simpler to resolve the problem with your friend’s laptop not connecting to your home Wi-Fi in comparison. Things I would look for would be:

    2. check in the router to see if it is only allowing access from specific types of Wi-Fi devices (i.e. N-only, A/B/G/N, B/G-only, etc). You may need to broaden this setting so that other devices aren’t being blocked,
    3. if this is a really old laptop it may be worthwhile to download and install the latest drivers for the Wi-Fi adapter. There could be an incompatibility between the Wi-Fi adapter and the router that is causing it not to connect,
    4. If the Wi-Fi network has been remembered on the laptop consider deleting it as the settings saved in it could have become corrupted.
    5. Hope that gives you something to go on 🙂

    • Lanz on February 19, 2013 at 02:44
    • Reply

    Such a great reply Boyd. I learned a lot. I already Bookmarked this page 🙂 Thank you.

    • Lanz on February 22, 2013 at 21:32
    • Reply

    Brilliant Byod. It works now. Thanks for the help

    1. Woohoo! Fantastic result 🙂

    • Andrea on August 22, 2013 at 05:07
    • Reply

    Thank you, I could not figure out how to set the exact parameters. Now it works like a charm!

    1. No worries 🙂

  • Great manual, thanks so much!

    • Lisa on April 2, 2015 at 00:02
    • Reply

    I just upgraded to 10.8 from 10.6 (yes, behind the curve by a couple years) and your fix worked like a charm after several frustrating hours of researching. Now have it bookmarked. Thanks for simple, easy-to-follow instructions.

    However, I seem to be unsuccessful in connecting to different wifi networks where I used to just unplug the ethernet cable and turn off Internet Sharing. We travel a lot so are left to get wifi from many different sources, either wifi or ethernet. Is there a small adjustment that can be made to ‘go back’ or do I have to completely undo all your 22 steps when I connect to, say, a wifi hotspot? Thanks.

    1. Unfortunately, you would have to completely undo all of the settings because they would more than likely be incompatible with other networks to which you connect. If you don’t undo the settings you won’t get any dataflow and websites and e-mail won’t work.

        • Lisa on May 22, 2015 at 21:53
        • Reply

        Thanks for the confirmation. It’s a pain, but at least I now know how to go back and forth. Been a happy Mac user since the MacPlus, but having more and more frustrations with Apple as of late. Spending more and more time searching for fixes such as yours. Their current trajectory doesn’t bode well so hoping they alter course soon. Thanks again.

    1. […] wasn’t able to do it rightaway. Wondered why. It has always worked. Then I came up to this post. Basically, all I had to do is to set the MacBook’s Wi-Fi IP address to a different first […]

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