I'm curious as to how or why I might want to take advantage of two ethernet ports on the back of the tower?

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    Mac Pro's are often used as servers. Can be great for isolating external network/internal network, and firewalling. – Harv Jan 29 '11 at 22:34
  1. If you wanted to connect to two different networks you could use this. So say you had an internal network which you had your own computers all linked to but were not connected to the internet, you could use one port to connect to this network. Then you could connect the other port to a broadband router/modem and access the internet on your Mac Pro. It's really more of an interest to businesses than domestic users.
  2. You could also use the second port to connect to Network Attached Storage (NAS) such as a MyBook World or similar. This would allow you to use the full bandwidth of one port for backups to disk while not affecting the speed on the other network.
  3. Backup interface in case one fails. You could have each interface connect to different switches on your network in case of a failure on one you still have access to the network on the other.
  4. There's also something called link aggregation which allows you to combine the ports to act as one port with twice the speed. However, this depends on your other network equipment supporting this mode.
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  • I do #1 (though not on a mac pro), and I'm a (power) home user. – Fake Name Jan 30 '11 at 7:23
  • Yeah @Fake_Name I do too but most users wouldn't go to the trouble and those that might probably don't need to ask this question – conorgriffin Jan 30 '11 at 14:12
  • Thanks for satisfying my curiosity! Of course, the link aggregation thing looks cool :) – mkelley33 Jan 31 '11 at 17:59
  • Link aggregation is very nice for speed and downtime considerations when a server needs to run fast and without interruption. – bmike Apr 22 '11 at 18:09
  • Incidentally, if you configure two ports such that they have the same IP address, you get failover even without link aggregation (provided that the system can tell one is down, e.g. is unplugged or the other end is unpowered). I've used this configuration to transparently switch between ethernet and wifi (from the same router) without losing TCP connections. – Kevin Reid Oct 16 '11 at 13:14

@Griffo Had some great ideas, I have one more:

Share your network connection with another computer. Usually, you'd use a router, but what if you don't have one, or it's misconfigured? You could connect your external network to one port and use network sharing over the other.

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Provide one of the ports to a Virtual Machine for bridged networking. This allows the guest VM to have direct access to the network on its own IP instead of sharing the host's network card via NAT.

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    You can do this in VirtualBox and VMWare Fusion with out a second card. One card, two IP's, bridge mode not NAT'd. – dhempler May 7 '12 at 2:51
  • Good point... My answer was kind of specific to my own use of Vagrant for setting up VMs... At the time it didn't support proper bridged networking, but it does now! – Andrew Vit May 7 '12 at 18:49

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