MacOS – Can’t access internet through network share

Short answer: you're doing it wrong.

The Buffalo device you've got is a wifi router. Normally this kind of thing sits between your cable modem (or whatever you've got connected to the wall) and the rest of your home network, and redistributes your Internet connection to multiple devices in your home.

If you turn on "internet sharing" on your Mac, you're essentially turning your mac into a wifi router as well. There is no reason to do this. And you're probably creating a "double-NATing" situation as well, which is undesirable. I'm not sure how you've got your network wired up physically, but that could be part of the problem too, I imagine.

So, in short, either get rid of the Buffalo entirely and rely on your Mac's internet sharing to do the same thing, or use the Buffalo to distribute your Internet connection and turn off Internet sharing on your Mac. I would do the latter. So your setup should look like this:

Wall socket -- Cable Modem -- Buffalo -- Computer
                                      -- Another Computer
                                      -- Yet Another Computer

This is the case both on a physical level and on a logical level—the Buffalo sits "upstream" of your computers in terms of networks traffic, whether it connects to them over an Ethernet cable or wifi.

To try to make things clear let's say you have 2 Mac named:

• Alex
• Bob

Here is the scheme of your small home network :

Internet --- USB --- Alex --- WLAN --- Bob

If when Bob try to join the wireless network (AirPort or WI-Fi or better WLAN) initiated from Alex, it gets an immediate rejection, this may be due to a network access with the wrong security setting.

To check this hypothesis, on Bob, simply go within:

System Preferences… > Network > WiFi > Advanced…

and look at the column Security.

If it is anything else than WPA2 Personal, double click on this network setting to fix it.

If it is already WPA2 Personal, double click on this network setting to check if the registered password is OK.