New Macs will not recognize ethernet

Apple > System Preferences > Network > highlight Ethernet in the sidebar > click the Advanced... button > in the TCP/IP tab, click Renew DHCP Lease > then click OK.

It also might help, if you turn off Wi-Fi and move Ethernet to the top of the Service Order List.

OK shifting gears back to the Mac Mini's.

What version of macOS Mojave are on? (Mojave 10.14.6 is the current version)

If they are still running on the factory installed version of Mojave, then perhaps you just need to update them to 10.14.6 or optionally start over and reinstall Mojave from scratch.

If that doesn't help, then call Apple Support and exercise your 90 days of free phone support.

Contact - Official Apple Support

Note that the fast Ethernet option is not compatible with some older Ethernet speeds.

Mac mini - Technical Specifications - Apple

"Ethernet

10/100/1000BASE-T Gigabit Ethernet (RJ-45 connector)

Configurable to 10Gb Ethernet (Nbase-T Ethernet with support for 1Gb, 2.5Gb, 5Gb, and 10Gb Ethernet using RJ-45 connector)"

This morning I hauled up one of the Mac Minis upstairs, where the modem, router, and switch are. I tried plugging the Mini into all three devices, directly - none of them worked.

I'm assuming that each time you connected this mini to each device, you powered-down all the devices before changing connections? That is, for example, when you tested the mini with the modem. You had both devices powered-down; switched the cables, and then, powered-up the modem first, then the mini ... correct? Just want to be sure that the modem or router was able to obtain the MAC address of the mini ... if it actually was able to.

One other thing I would try, before taking them to your local Apple Store, is to use the Terminal app and try a few commands & booting up your mini in Safe Mode.

Terminal

Connect one of the minis directly to the modem. Run the Terminal app, and then, enter the following command:

• ifconfig en0 <enter>

The following image is an example of the results I just got for my 2018 Mac mini:

Two things to look for:

1. In the "flags" line, note the value "UP" <-- This would indicate that the Ethernet port is "up and running."
2. In the "status" line, note ghe value "active" <-- This would indicate that the Ethernet port is enabled.

In this example, my mini is equipped with a 1Gbps Ethernet port and I have purposely enabled an MTU value of 9000 for the connection between my mini and my Ubiquiti managed switch to use Jumbo Frames.

If you do make the trip to the Apple Store I will be curious to the results as I still suspect that this may just be a switch (or router) misconfiguration and not any of your Macs. Of course, anything is possible.

There are two stages to using an Ethernet connection.

1. The physical link negotiation
2. Obtaining a TCP/IP address

I am presuming you are using DHCP so the second stage should be automatic. However let's first concentrate on stage 1. If you plug an Ethernet cable in to the Mac mini and the other end is plugged in to a working Ethernet switch then the Mac mini should report the state as connected but it might report that it is using a self-assigned IP address because it did not get a response from a DHCP server. So the three possible states are -

1. Connected (physical connection and a TCP/IP address)
2. Self-assigned IP (physical connection no TCP/IP address)
3. Not Connected (no physical connection)

Which state is shown for your Mac mini Ethernet connections?

Usually 'Not Connected' would be down to either a bad Ethernet cable or a problem with the Ethernet switch e.g. not turned on. Bad connections could be down to some of the wires being not connected or connected in the wrong order. There are eight wires in four pairs and these are supposed to be wired in a specific order at both ends.

Whilst the Mac has (sadly) no flashing LED lights to show the status of its Ethernet socket normally the Ethernet switch will have, you should see if any light up corresponding to the socket the Mac mini is plugged in to.

I would say that normally a MacBook and a Mac mini would be happy with the same Ethernet cable wiring. (Except for 10Gbps connections.)

In addition to John's comments, another command that may help is:

• sudo ifconfig en0 media 1000baseT mediaopt full-duplex inet up

If this works, it will set this interface to 1Gbps. To set it to 10Gbps, you would enter media 10Gbase-T mediaopt full-duplex instead.

Sounds like either your Internet Service Provider is controlling the addition of new devices or your router security is blocking them. At this point, I can only suggest that you call your ISP and have them help you with the problem.

If it works when connected directly behind the router bypassing your home wiring, you’re out of luck at least as far as Apple is concerned because that’s what they’ll check. So do it before looping them in. What you’ll then need to do is to install a router in bridge mode to your room wall and connect the mini to it. As to why older Macs don’t need bridges, good question, but unfortunately pointless in your situation. Good luck.

Based on the response from Malcolm the Mac mini 2018 can either support -

10Mbps, 100Mbps, and 1Gbps

or

1Gb, 2.5Gb, 5Gb, and 10Gb

It cannot support for example 10Mbps, 100Mbps, 1Gbps and 10Gbps

Therefore if all your Mac minis are equipped with the 10Gbps Ethernet port and you are connecting them to a network switch which only support a maximum of 100Mbps or are using a cable only capable of 100Mbps then this could explain your problem. I have never bought a Mac mini with a 10Gbps capable port so I do not know exactly how to tell the difference but it would be worth looking in Apple Menu -> About this Macintosh -> System Report -> Networking and checking to see if all your Macs are the 10Gbps model.

Can you indicate what make and model of network switch you are plugging into?

If you plug the MacBook in using the same cable to the same socket what speed does it report in System Preferences -> Network -> Ethernet?