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I need to transfer a lot of files between two 2019 MacBook Pros, but when I connect them using a Thunderbolt cable, the bridge never connects and assigns an IP via DHCP.

I tried manually assigning an IP and subnet mask to both of them, but still can't connect them.

What is wrong here?

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    What values did you use for the IPs and subnet?
    – benwiggy
    May 10, 2021 at 20:08
  • 192.168.2.1 and 192.128.2.2 with subnet 255.255.255.0 May 10, 2021 at 22:13
  • @ShrewdSimian That second IP is in a different subnet (128 instead of 168). Typo? Also, is the "Thunderbolt Bridge" showing as "Connected" in System Preferences -> Network ? May 11, 2021 at 17:48
  • Yep...typo. meant 168 on both. No, it is not showing connected. This is the problem I am having....can't get it to actually connect. May 12, 2021 at 4:21

1 Answer 1

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Using a Thunderbolt Bridge with Internet Sharing

This is an example where a Thunderbolt bridge was be setup to transfer files. This was done using legacy Thunderbolt. I assume the same procedure would be used with Thunderbolt 3. The host computer is connected to the internet through ethernet using a DHCP assigned IP address of 192.168.0.88 and subnet mask of 255.255.255.0, so I setup the Sharing pane of System Preferences as shown below. You could also use Wi-Fi.

I have a Thunderbolt 2 cable connecting the two Macs. On the client computer, I setup the Network pane of System Preferences as shown below. I needed to select the Renew DHCP Lease button. The client is current not connected to the internet by Bluetooth, Ethernet or Wi-Fi.

I setup the Sharing pane of System Preferences on the client as shown below. Note: In my case, the image shows I can connect to the client from the host by using smb://192.168.2.2.

From the menu bar of the Finder application on the host, I can select Go->Connect to Server... and enter smb://192.168.2.2, as shown below.

Proceed by selecting the Connect button.

Using a Thunderbolt Bridge without Internet Sharing

Note: Here the IPv4 addresses of 192.168.2.1 and 192.168.2.2 were used because these were the addresses chosen by the host in the internet sharing example. If you use these address, then you must make sure there will be no conflicts with other LANs being used by either of your Macs.

I suppose internet sharing is not required. For example, the computer, referred to as the client, could have been manually setup, as shown below.

And, the computer, referred to as the host, could have been manually setup, as shown below.

With the above configuration, the Sharing pane of System Preferences, for the computer referred to as the host, could have internet sharing turn off, as shown below.

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  • Instead of using 192.168.2.x where x is a low number that might collide with other devices on the 192.16.2 network, choose a larger number which won't collide. Simply counting up the number of devices which regularly connect to the network, and adding 10 to that value will give a little headroom. Nice explication of the methodology of using Thunderbolt bridges, BTW.
    – IconDaemon
    May 11, 2021 at 16:04
  • @IconDaemon: With a Thunderbolt bridge, there are only two devices on the LAN. Both devices being the Macs at each end of the Thunderbolt cable. May 11, 2021 at 16:45
  • Thanks David. If one of the Macs is connected to 'real' network, won't there be a problem? My router uses 192.168.2.x. Thats the source of my now meaningless comment. :-)
    – IconDaemon
    May 11, 2021 at 19:07
  • If your router uses 192.168.2.x and the host Mac had internet sharing turned on, then the host Mac would choose something other than 192.168.2.x for the Thunderbolt bridge. If internet sharing is turned off and configuring manually, then you would have to choose something other than 192.168.2.x for the Thunderbolt bridge. May 11, 2021 at 19:54
  • Thank you. This was far less plug-n-play than I expected from Apple. For some reason I remember this just working last time I needed to do a transfer outside of the Migration Assistant. May 12, 2021 at 18:30

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