If you use more than one type of network connection with your Mac, or if you access more than one Wi-Fi hotspot, then you might find one connection being preferentially used over another, which can sometimes frustrate your ability to get online or transfer data between systems. For example, if you generally use Wi-Fi for connecting your Macs to the internet but you try using an ad-hoc link with an Ethernet or FireWire cable for faster data transfers between two systems, you might find the system still uses Wi-Fi for the connection, even though the alternative one is configured properly.

This happens because OS X prioritizes network connections, so regardless of the number of active connections, when you try to establish one from your Mac, the one with the most priority will be used. In the case above, if Wi-Fi is given more priority, then as long as it is connected and provides a route to the second Mac, OS X will use it to establish the connection, even though the faster Ethernet connection is available.

There are two easy ways you can get around this, and if you use Wi-Fi, you can also use prioritization for your Wi-Fi hotspots to ensure you’re always connected to the one you want.

Temporarily disable all but the desired connection

If you wish to connect to your second Mac, then you can connect it via FireWire, or another connection of your choosing, and then disconnect your primary network connection. If you primarily use Wi-Fi, then you can do so via the Wi-Fi menu on your Mac, or if you use Ethernet then you can simply unplug the cable. Once this is done, then OS X will switch to use the next available connection, which will be the temporary one you have set up.

After you are done with your file transfers, you can re-enable or plug back in your primary connection, and be back up and running.

Prioritize network connections

If you open the Network system preferences, then you will see a list of your available connections to the left. In this list, the top-most connection is going to the the preferred network used (you should see this list re-order itself based on whether or not a connection is active). For any active connections that appear green, since the first one listed will be given priority, you can configure the system so those you only use temporarily are at the top of the list.

For example, if you always use Wi-Fi, but every now and then use Ethernet as an ad-hoc connection between your Mac and another one, then you can set Ethernet to be higher priority than Wi-Fi. With this setup, your Mac will connect via Wi-Fi, but when you have an Ethernet cable attached it will try to use that connection first.

To do this, go to the Network system preferences and then click the small gear menu under the network connections list, followed by selecting the option to set service priority. In the panel that pops up, you can then drag services above others to give them priority.

Note that unlike disabling connections, this setup is set-and-forget, where you simply have to attach the cable or otherwise establish the connection for it to be given priority and work. Disconnecting will then revert to your alternative connection being used.

Prioritize Wi-Fi hotspots

Another situation you might find yourself in is if you have several Wi-Fi networks nearby that you have recently connected to. If one is relatively slower and poorer quality, then you might wish to only connect to this if another more preferred connection is not available. OS X should automatically learn which Wi-Fi connections you prefer, and give those priority; however, you can ensure the connections are made how you want by manually prioritizing the Wi-Fi connections list:

1. Open the Network system preferences and click “Wi-Fi.”
2. Click the Advanced button at the bottom of the window.
3. In the Wi-Fi tab, locate your preferred Wi-Fi networks and drag them higher in the list than others you do not prefer.

Note that in this setup, you do not need to drag your preferred connections to the absolute top of the list. Instead, you simply need them to be above those you do not prefer. This may make organizing this list easier to do, especially if you use a MacBook and travel, and have a number of different wi-fi connections listed as a result.

UPDATE: Clarified instructions on how to set network service priority.