Two Ethernet connections possible on MacBook?

Can a MacBook be connected to two ethernet networks at the same time? Would it hurt anything to try? Obviously, the ethernet connections would be through USB adapters and hubs.

One ethernet connection would provide internet access; the other would be connected to a Time Capsule with a printer attached.

I know the Time Capsule could provide internet access if I connected the ISP device to it. I don’t want to do that. At present, the ISP device connects to an Airport Express and I get internet by Wi-Fi.

Are you asking about more than 1 access to the Internet? or access to more than one Ethernet enabled device?

I’m probably wrong but I think your MacBook will only use ONE Internet connection at a time. It can / will switch between them but only one at at time

No, one ethernet connection would provide internet access. The other ethernet connection would provide access to a Time Capsule and a printer (connected to the Time Capsule).

I don’t think that would work, but I also don’t think it would hurt anything to try it. My guess is that it just wouldn’t work… or (what’s worse) it would work unreliably.

I would expect it to work. It won’t hurt anything to try.

Sure, it should work. Make sure to configure both interfaces and put them in the order of priority you want.

@Will_M I’m dying to know if this does work once you give it a try. I can’t see how it would, given that I think macOS will use the first network adapter for all Ethernet traffic, but I’m happy to be wrong.

Slightly different details, but fundamentally same idea: running two Ethernet interfaces simultaneously on one Mac.

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Doh! Of course—the two adapters need to be connected to completely separate Ethernet networks.

OP here. The short story is that it doesn’t work, at least not yet. The caveat is that it could well be because I’m not smart enough.

The device that my ISP calls a modem has two ports that I will call ethernet ports. One is connected to an Airport Express, and that is how I get internet. I just connected the other port to an ethernet to USB adapter and plugged it into a USB hub. I got a notification that there was a new device and I should configure it in the Network control panel.

In the Network control panel, I clicked the + at the bottom of my networks and selected “USB 10/100/1000 LAN (en6)” from the dropdown menu. (I was also offered “USB 10/100/1000 LAN (en3)” from the menu; “USB 10/100/1000 LAN” is the name of my other connection.) I left untouched the “Using DHCP” choice for Configure IPv4. After a while, the indicator changed from red to orange. Under Status: Connected, it said that the connection had a self-assigned IP address.

I switched the ethernet cables (ISP device and Time Capsule). After a short while, the new network had a green indicator and the old network had an orange indicator and the message about a self-assigned IP address.

I told Time Machine to back up to the Time Capsule and the backup started. As I was typing this, I got a message that the backup failed. I switched the cables back and told Time Machine to back up to the Time Capsule. The backup started and shortly thereafter I got a message that Time Machine completed a verification of my backups and (to improve reliability) it must create a new backup for me. I seem to get this message about every six to eight months, and it seems like it was due, so I don’t know if this is related to the “two ethernet networks” scenario or not.

I disconnected the Time Capsule ethernet cable from the adapter and told the new network to Renew DHCP Lease (in the Advanced… pane). After a short wait, it had a self-assigned IP address. I reconnected the Time Capsule and made the new network inactive.

I would fiddle with it but it seems more productive to ask here for suggestions, especially for configuring the new network. Thanks for any insight.

I have to admit, I’m confused how your setup works.

Does your MB have an Ethernet connection at all? I understand you’re connecting to WAN over wifi via an AP Extreme connected to your ISP modem. To me right now it sounds like the second LAN port on that modem is connected to a USB hub via Ethernet-USB adapter. What for? Where is the Time Capsule in all this?

How about this, connect the Time Capsule’s WAN port to the USB adapter and plug that into your Mac, not the ISP modem. Continue getting WAN access through your Express’ wifi, but connect to the TC over Ethernet (via USB) configured as the 2nd network interface. It will still get a local IP since it sees no upstream DHCP, but that shouldn’t matter because you only need local access (from your Mac) to it for TM to work (thanks to Bonjour IIRC).

Another idea is to simply use the Time Capsule instead of the Express as the device hooked straight up to the ISP modem (use the TC’s WAN port for that). If you don’t want the TC offering wifi you can always shut that off, and instead hook up the Express to one of the TC’s LAN ports. Then serve up wifi through the Express as you do now. That way you should be able to access the disk in the TC for backup locally, but internet access will still occur over wifi served from your Express.

I don’t blame you. I’ll try to describe it. The MacBook, when in my office, is connected to an Acer display, which has two USB ports. The Time Capsule connects to one; a USB hub with external disks, keyboard, lightning cable for iOS devices, and maybe other stuff (I’m away from there just now) connects to the other. I connected the ISP ethernet to the USB hub (but it appeared that switching the two ethernet connections had no effect). I have been getting internet by Wi-Fi, and I wanted to explore getting internet on the second ethernet connection.

A primary reason for my setup is that I only connect to the internet when I need to. In other words, I want to be able to disconnect from the internet (but stay connected to the Time Capsule and the printer) when I am not doing internet things. At present, I turn Wi-Fi off. If the second ethernet worked, I would make that connection inactive.

Did that clear things up? I apologize if I should have included that information at the start.

When you say you’re connecting the TC to the monitor’s USB, you mean over a Ethernet-USB adapter, right? The TC has a USB port too but that’s not for networking.

If you’re connected to the internet over wifi, you only need one Ethernet connection to your TC, right? In that case you don’t need to connect the ISP modem to your hub.

You can absolutely connect to the internet over wifi and use a TC via Ethernet at the same time. At my desk I’m connected to wifi and Gigabit at the same time and both work simultaneously, because I know I an reach a locked down host which can’t be reached over wifi (i.e. it’s coming in over Ethernet) while I can also reach TidBITS at the same time which obviously comes in from the WAN over wifi because my Ethernet does not allow connections to the WAN for security reasons.

Monitor USB hubs are notoriously flaky. For troubleshooting purposes I’d try disconnecting from it and hook up only the TC via USB-Ethernet adapter straight to your MB, while using wifi to access the Internet. See if you can get that to work.

Yes, the ethernet connection is through an adapter to USB, both for the Time Capsule and the ISP.

Yes, I only need to connect to the Time Capsule by ethernet (both for Time Machine and the printer). That is my current setup, and I do get internet by wi-fi. I’m sorry if that wasn’t clear earlier.

The Time Capsule connection by ethernet through a USB adapter connected to the monitor does work. Connecting directly to the MacBook’s only port has its own set of problems, like being unable to charge the battery or access external disks. (I would be interested to know if Time Machine wanting to scrap my backup and start over every six to eight months is related to the monitor’s USB hub, but that event is rare enough that I don’t know how I would determine anything conclusively.)

Repeating what I have tried to say, I currently get internet over wi-fi and Time Capsule over ethernet. There is no troubleshooting required. What I would like to do is get internet over a second ethernet connection. (Why? Mostly because I’m curious, but also because the ISP says there will be better throughput.) The second ethernet network connection shows up in the Network Control Panel but with a self-assigned IP address. If I swap the two ethernet cables (going to the two ethernet-to-USB adapters), the Time Capsule ethernet works and the ISP ethernet does not, just as before. This would be consistent with Adam’s belief that macOS will use the first network adapter for all ethernet traffic (and that it remembers the network even if the hardware connection changes).

If you have a choice, I would suggest that since your Ethernet connection will always be faster and more reliable than WiFi, that you should use it for Internet access since both TimeCapsule backups and printing require relatively slower connection speeds.

If you just want internet over Ethernet, you could always replace the Express with the TC. The TC has LAN ports you can connect to from your MB’s USB hub.

Macs can connect to two Ethernet controllers simultaneously. There’s ample documentation on that including Apple’s own re: MP with dual Gigabit controllers. Now your situation might be different because of details related to how Bonjour works with the TC and the USB adapters, though I doubt the latter presents a fundamental issue.

It should absolutely work. Just make sure you use a different IP range on each interface.

The only configuration trickery might come from what happens with default routes: you’d only want one to be set on the interface that actually gets you to the internet, and so you might find it easiest to use a fixed IP configuration (no DNS, no default route) on the private network interface configuration and let DHCP take care of the Internet facing one as usual. Use the “Locations” feature of the Network Configuration preferences (I assume Catalina has it, even though it’s not had any love since OS X 10.0], and you can make it easy to switch back to Automatic if using networks elsewhere.

(By default Mac OS will happily let you have the same IP address on more than one interface, or just different IP addresses on the same or different subnets, and then it can use their priority when it decides on which interface to send packets out of e.g. use an Ethernet if plugged in else use an active and associated Wifi. But that’s obviously not what you want, here)

Printing is certainly low bandwidth, but given that most people have at most 100 Mbps for Internet connections, wouldn’t it make sense to give the Time Capsule backups the most bandwidth?

What I would suggest instead of this complicated setup is to use the Time Capsule as your internet router. Use a device like this between your ISP’s service and the Time Capsule:

This will allow you to turn internet service on and off to meet your needs. Turn it on when you want service, off when you don’t, and this will allow you to connect with other devices on the internal LAN (such as the Time Capsule for backup and the printer) even when you have the internet service turned off. And that will allow you to connect the Mac to one of the ethernet ports on the Time Capsule.

It is rare that an ISP will provide more than a single IP address to an end user, so that is why you cannot connect their incoming service to a switch or hub and split the connection - only one of the devices will get the IP address from the ISP.

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Two simultaneous Internet/Ethernet connections work just fine. On separate adapters, of course, and with different subnets. I do this all the time on my 16-inch MBPro running Catalina.

You can select the service priority in Network Settings. In my case I have two separate networks/routers that I don’t want to interact. One of them is a Ham radio based network that doesn’t allow encryption (no HTTPS), and I have radios connected to it. In case of disaster, it should be more robust than consumer Internet.

But I want to monitor both networks from my laptop. Hence, the two connections.