Sudden ethernet port failure - unknown cause

Are your Wi-Fi and wired networks in separate IP subnets? If not, then your Wi-Fi is probably best configured as an access point. Which might be easier in general.

They should be in separate subnets here, or IP routing can get confused. You’re running two separate connections with a router between the two.

The alternative here is to remove the router; to switch the router into an access point configuration. That AP configuration can be simpler, but not all Wi-Fi devices support that, and it can mean certain functions of some Wi-Fi device services might no longer be available. Conversely, network services on the wired network become immediately a ailable on the Wi-Fi.

Subnet routing is a part of having multiple network segments and multiple network controllers, when those segments are disjoint and separated by IP routers. Without different subnets, the IP routers have no means to determine the network segment to forward the packets toward.

Based on a quick lookmaround the ‘net for descriptions of subnetting...

https://www.think-like-a-computer.com/2011/08/24/ip-routing/

There can also certainly also be problems with cabling and cable failures, with Wi-Fi interference, and other issues, but this network is incorrectly segmented and the most problematic system is the one that’s connected across separate network segments.

Try connecting to just one segment at a time, as a test.

The Wi-Fi network is connected to a router, so everything on that Wi-Fi is one subnet and one segment.

The Ethernet is connected to a router, and everthing on that is one subnet and one segment

Routers route between segments, and routers use subnets to determine what’s local and what‘s not.Specifically, networking in each connected device effectively asks itself the following when sending a packet... “is this destination IP address on the same segment as I am, or is it on a different network?” If it’s on the same segment (same subnet), it is sent directly.... If not, the packet is set to the default router. On a Wi-Fi network, the router is the Wi-Fi device. The router than determines what the next hop will be. This process repeats for each packet, separately. Again, each of these packet routing decisions are being made in isolation and utterly separately. There is no consideration of which NIC sent the packet in any response, and if there are multiple NICs then any or all of those paths can be used, that per-packet NIC link selection decision further often depending on link costs and link data loss counts and related details.

... If you’ve not already done so, please read that linked article.p, and the two related articles referenced over there. That’ll get you some background...

Now as for the wired network, I’m here assuming that the Wi-Fi is connected to that same network, and that the wired network is then connected to some ISP gateway. There are numerous other ways to configure this, but that’s one of the most common.

As for how I’d set this up... Two segments means two subnets, or it means reconfiguring the Wi-Fi router as an “access point” or—as that’s also sometimes called—a ”bridged“ device, and which makes the wired and wireless networks (logically) into one network segment and thus needing only one subnet; it disables the routing functions.

If they’ll let you, switch the router to a bridged mode, and configure your own firewall and subnets behind that. That gets their firewall-router-gateway box out of the mix.

More than a few folks have has tussles with how AT&T have set up their own internal customer network, and how that design and their firewall-gateway-router box then configures and manages the local network.

Hopefully the AT&T folks get that all sorted when they transition their internal network and their customers fully over to IPv6.

If you turn the computer WiFi off, does it work using Ethernet?

True. I only was asking as a test. If it works, it isn't a port failure or a cable failure.

AT&T Uverse has a notoriously perverse networking configuration, if that’s what you have here.

Eric Root wrote:

If you turn the computer WiFi off, does it work using Ethernet?

Good for testing and a good suggestion here, certainly. But a caveat, as a potential workaround or as a solution: some of the newer macOS features can require Wi-Fi enabled. Watch-based logins and AirDrop, among those.

None of the other cited devices have multiple NICs active in parallel. As soon as you have multiple NICs active, you’re configuring IP routing and disparate local subnets for each local segment that the router is connected to, or alternatively you’re reconfiguring to make your network flat and to remove that routing and configuring (or maintaining) a single subnet across the network.

IP doesn’t automatically return traffic in the NIC that transmitted it—unless there’s just one NIC—nor based on the route selected by thr previous or subsequent packets in a stream of packets, as all of the routing decisions are made by the intervening routers.

You have local IP addresses that are potentially unreachable, depending on which route is selected for the IP network packet.

IP also doesn’t respond at all well to flaky connections. The back-offs secondary to packet loss can massively impair network throughout.