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We have deployed Thunderbolt Displays in my organization, which is almost entirely Mac-based. As you may already know, Thunderbolt Displays have internal Ethernet adapters which is very convenient in that a user can now connect their wired network connection to their display rather than their MacBook, and when they come in to work in the morning, the only thing they need to connect to their MacBook is the MagSafe adapter and Thunderbolt cable, both which come from their display. Essentially, the display is now acting as a docking station with all of the most commonly used ports on the display sent over the Thunderbolt connection to their MacBook.

However, this convenience presents a problem on more heavily managed networks like my own (e.g. enterprise) that use DHCP reservations.

Traditionally, when a new Macintosh comes in, a system administrator would add the MAC address of the wired ethernet port on the Mac to our DHCP server, and associate it with an IP address. That MAC address will now always get the same IP address assigned to it via DHCP. This is a tried and true method employed on most enterprise networks, regardless of operating system/platform (this all happens at the network level). When that MacBook connects to my network via the wired connection, it will now always get the same IP address – the one that I've designated in DHCP.

Apple has set a new expectation of using the Thunderbolt display as a docking station. This is convenient, but it introduces a problem. Now that Thunderbolt displays have on-board ethernet with a MAC address on the display itself they are the connection point to the network. It would be easy to start adding reservations for displays to our DHCP server, but ultimately foolish because it wouldn't gain us anything in manageability and would only serve to confuse us or slow us down as systems administrators when we're trying to access client workstations for remote support and administration.

I want DHCP reservations to still get honored for the Macs on my network, even if they are behind a Thunderbolt display. This way users can move around and plug their MacBooks into any of our Thunderbolt displays and still get the same hostname and IP address from DNS and DHCP, giving them consistency, and making administration as straightforward as it once was.

So I ask you, fellow Mac administrators. How would you handle this situation?

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  • I believe the DHCP-request also includes the name the host thinks it had. Perhaps you need to use that instead of the MAC address.
    – Thorbjørn Ravn Andersen
    Jul 12, 2012 at 14:57
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    Why not set up your reservations based on a client ID than a MAC address? The real question is what else are you doing systematically based on a fixed IP address. Without knowing how you manage things / how this trickles down it's hard to advise you. Why not use ARD and just let the IP addresses fall where they may?
    – bmike ♦
    Jul 12, 2012 at 17:51

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This depends a lot on your DHCP server, but I would look into using the client id to assign static IPs. It's not tied to the individual hardware (although you'd have to set it at least once when they first use a TB Display), so it avoids the switching displays problem. It can be set via the networksetup tool.

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  • So this means that the MacBook would dictate the Client ID assigned to the Thunderbolt Display? In other words, would that mean if on my machine the client identifier for the Thunderbolt interface is set to "stuntmachine", and on my friend Luke's machine, "lukewarm" is the client identifier of the Thunderbolt interface, and we switched, his monitor would now be getting "stuntmachine" as a client identifier and vice versa?
    – user23645
    Jul 12, 2012 at 15:46
  • Yes. The catch is that it's a per-interface setting in Network Preferences, so it needs to be set once for each interface you intend to use. I'm not sure if each Thunderbolt Display you plug in would add a separate interface to Network prefs or not, but I suspect with some scripting you could figure out a way to have it automatically set.
    – robmathers
    Jul 12, 2012 at 15:57
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Simply assign a static IP to the Network Adapter of the screen and remove the reservation.

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