Q My 3G Internet connection works through a MiFi. When up and running, speeds are good, but despite the modem showing a full signal and its browser interface claiming the connection is good, it periodically throws my MacBook Pro or Mac Pro off. What is going wrong?
A The most immediate problem (revealed in screenshots sent) is that you have two different devices connected to your MiFi with the same IP address of 192.168.1.100. One has the MAC address of an Apple device, presumably your MacBook Pro, whilst the other has a MAC address from a different vendor, perhaps a third-party WiFi interface in your Mac Pro.
Running two devices with the same IP address on a single network will cause problems, including sudden disconnection, and must be avoided. Check whether both devices are being assigned this address by the MiFi’s DHCP server: if they are, then the server is malfunctioning, suggesting that the MiFi has a fault.
You can work around a duff DHCP server. As yours is handing out addresses starting 192.168.1., you know that is where it is expecting to find local devices. On each of your Macs, open the Network pane and change the IP setting to Configure IPv4 Manually. Then assign them unique addresses such as 192.168.1.20, .21, and the like. These should be safe, as the MiFi uses the address 192.168.1.1 on your local network. It is possible that other devices such as printers will need to be given manual addresses too.
Once you have got stable and compatible IP addresses, see if the disconnections disappear.
If they continue, you need to establish whether they too are the fault of the MiFi. The simplest way to do that is to have both Macs up and running; when the line appears to drop on one, see whether the other Mac can still access the Internet. If it can, the problem may rest with the networking hardware in the disconnected Mac rather than the MiFi itself. If both Macs lose the Internet at the same time, you definitely need to get your MiFi replaced.
If fixing the IP addresses resolves your problems, you could look for a firmware update, or some means of performing a ‘soft’ reset of the MiFi. That could reboot the DHCP server and get it to work properly again, perhaps.
Updated from the original, which was first published in MacUser volume 29 issue 2, 2013.