How and When to Use Both Ethernet Ports on a Mac Pro

| How-To

Unlike all other Macs, the Mac Pro line has two Ethernet ports. How and when would one want to use both of these ports? I'll discuss a pair of typical scenarios.

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There are several reasons why one might want to utilize both of these Ethernet ports. This article is focused on the ones I'm familiar with and have tested. Other configurations include, for example, something called "link aggregation." That's when two Ethernet ports are combined to achieve greater bandwidth. That's a subject for another time.

What I'm going to discuss here is a connection to two networks, simultaneously. That's a situation that doesn't usually arise in a home, but it could. It's more likely in a small office.

If course, at home, some typical configurations are:

  1. Cable Modem > Router > Mac
  2. Cable Modem > Router > Switch > AirPort, Macs, iPads, etc.
  3. Cable Modem > Wireless Router with ports > Macs, iPads etc.

Those scenarios are generally reflected in the top half of Figure #1 shown below, using just one Ethernet port on a Mac Pro.

Figure #1

However, it may happen that a home or, more often, a small business may have two separate networks. For example, one might be a production and the other an experimental network (or a back up). Or two small, co-located companies may be sharing resources. Now, let's look at the bottom half of Figure #1 above.

Option #1. (Dual Networks) In this case, the Mac Pro is connected to two separate networks, both on the Internet, and each Ethernet port has its own IP address. I've chosen to use two of the most popular private (non-routable) IP address schemes, 10.x.x.x and 192.168.x.x, in this example. The Mac Pro can access everything on both of those networks at the same time, and devices on those networks can see the Mac Pro.

For example, here's what the Mac's System Preferences > Network might look like for port #1 (called Ethernet).

Figure 2.

And here's what it might look like, in parallel, for port #2 (called Ethernet 2). In this case, a second IPv6 network is being used for testing.

Figure #3.

If there are times when there's a need for security, to isolate the Mac from one of the networks, that's managed with the geared popup at the bottom of the Network window. Select "Make Service Inactive."

Figure 4.

One example of how this would help is the following. Suppose Ethernet port #2 were not in use on the Mac Pro in Figure #1. Further suppose an AirPort base station is connected to switch #2, but the Mac Pro is acting as both the master iTunes library and as print server on port #1. Connecting the Mac Pro's port #2 to switch #2 allows all the Macs, iPads and iPhones connected to switch #2 to print with, for example, Printopia. Plus the iPad and iPhones can then do their Wi-Fi sync to iTunes.

This is just a simple example of how a Mac Pro can bridge two networks and provide simultaneous services.

Option #2. (Isolated LAN). In this case, the Mac Pro is acting as a file server. It has access to the Internet via the router on port #1. However, port #2 is connected to a LAN of Macs (and perhaps PCs) that use the file storage capacity of the Mac Pro server, (and perhaps other services) but they don't have access to the Internet. Some design/publication studios use this technique to isolate their production work from security risks coming from the Internet.

Because there is no router that invariably has a DHCP server when using Option #2, the Macs on that LAN have two options to obtain a unique IP address. 1) They can be manually assigned or 2) OS X client does have a built-in DHCP server capability. Note that OS X Server has a better GUI for managing that DHCP server.

We're done.  But for those without a Mac Pro, there is an intriguing way to achieve the same thing.

An iMac Alternative

This raises the question: what if one is using two networks in a small office or home and can't afford a Mac Pro? iMacs only have one Ethernet port. But they save the day by also having Wi-Fi — which can also be used simultaneously.

Again, referring to Figure #1 above with, say, an Apple AirPort base station connected to switch #2, Turning on the iMac's Wi-Fi in addition to the the existing Ethernet network on port #1 (the only port) would allow access to the second network via switch #2.

One can add to the existing Ethernet configuration by using the "+" button at the bottom of the Network window. Use the popup labeled Interface: and select Wi-Fi.

Figure 5

This mixed mode isn't as secure as using Ethernet cables, but it works.

Summary

The ability of a Mac to operate on two separate networks and have a simultaneous pair of IP addresses opens doors. Flexibility is introduced for service isolation, security, or redundancy. It's a really nifty capability of our modern Macs.

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My thanks to Jim Tanous who contributed to this article. Mac Pro image via AnandTech.

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Comments

Bosco (Brad Hutchings)

You could add another Ethernet port to the iMac via a USB dongle. $20-ish on Amazon.

John Martellaro

Brad:  Good point.  I’ll have to test that as well.

urapns2

I was using USB to Ethernet for second Ethernet port. Went from 10.6.x to 10.9 and USB Ethernet is no longer supported.

krventh

You can also add a second ethernet port to recent Macs using a thunderbolt to ethernet adapter cable.

I added a second ethernet port to my mac mini server to connect to a NAS using iSCSI (specifically a LaCie 5Big NAS).

Imagestealer

I use this type of connectivity in my work all the time. 

I have an rMBP that connects to the internet via WiFi, and an internal LAN that I use for automation control software development and testing.  Works very well for my purposes, as I can get my e-mails via the Wireless connection and do all the other activities via the Wired side. 

This works well both in a Boot Camp or Parallels environment.  Windows is needed as all the Automation and Control software is windows based, and all the communications stuff like e-mail and VPN is on the OS X side.

Marcelo Navarro

Dear John,

Thanks a lot for your post, but I still have a doubt… I d’ like to know if the following setup is possible:

I have one MACPRO, i’d like to connect it to 2 iMacs (lets call it iMac1 and Imac2) using its 2 ethernet ports…. I just connected it as iMac1 -> MACPRO -> iMac2.

My MACPRO sees both iMacs, but iMac1 only sees MACPRO, also iMac2 only sees MACPRO, otherwise, iMac1 does not see iMac2 and iMac2 does not see iMac1…

Is it possible to config MACPRO to work as a “bridge”, this way, every computer sees each other ? I’d like iMac1 to see Mac Pro and iMac2, the same for iMac2…

thanks a lot!!!

P.S. Sorry about my english, to write the post so extended, I’m from Brasil smile

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