Network Issues

Well I got my answer from the Apple repairer, and it was very simple.

I changed the Location (see the first pull down in the first screen shot above) from Bigpond Broadband ADSL to Automatic, and then the computer found the IP address on boot properly on boot.

When I looked at the actual settings within the location, I could only find a different WINS name, which I can't see how that would have affected the finding of an IP address.

Anyway, thanks for all your assistance, but it was a very simple solution staring me in the face from the very first line from the beginning!

MrHoffman wrote:

I see three hosts with the view from one computer (.2, .3, .5), and two hosts (.2, .3) from the other.

I’d expect to see a response from the modem, and from each other connected device.

Run an audit of your network.

Figure out what’s at each IP address, and how each is connected.

What’s using .5?

What’s the wiring or the Wi-Fi network layout?

I had earlier removed Little Snitch using its Uninstaller.

The network layout is a modem-router connected through a gigabit switch to the iMac, and to the MBP normally directly.

• .5 = MBP

Gigabit Switch with:

• .2 = iMac
• .3 = Printer

I use the switch purely for getting the right physical proximity to connect cables that because of where the devices are located won't reach to the modem.

So, after all these tests, I can't replicate the problem of the ethernet port simultaneously getting/not getting an IP address from boot on any other machine, only the iMac. So, I rang Apple Support, and they booked the iMac into an authorised third party repairer. I'll tell you how I get on.

Ezequiell wrote:

Have you tried removing all the interfaces you have added?

I see you have USB modem 2, and Ethernet 2, etc...

Remove all the interfaces you aren't using, including the Ethernet one, and leave WiFi alone. Then re-add the Ethernet interface again (Should be named just Ethernet and see if the behavior remains the same.

Done. Rebooted. Results:

Looks good ....

but Network Utility remains the same:

BDAqua wrote:

However I had no internet connection and pinging using Network Utility failed for everything beyond my router.

With a Static IP try this...

System Preferences>Network, click on the little gear at the bottom next to the + & - icons, (unlock lock first if locked), choose Set Service Order.

The interface that connects to the Internet should be dragged to the top of the list.

....

Apply.

Done. See above.

Working through the rest of all your kind suggestions...

MrHoffman wrote:

With the ping, I’m not asking to ping beyond the router, I’m asking to ping the local network.

192.168.0.255 is a special multicast address.; local broadcast.

....

So, the result from another machine on the network:

When I disconnect and reconnect the ethernet cable on my iMac so that I get connected the ping now works, but now it finds: 192.168.0.10 (10 times) 192.168.0.105 (three times) and 192.168.0.130 (9 times).

What’s on the other end of the Cat 6 cable? The firewall/gateway/modem? Or a network switch?

A Technicolor DJA0230 Modem router. I have two, an old one which was initially connected, and an unused one (my telco helpfully sent one to our farm, which is about 9 km from the exchange, and which can't sync, obviously!).

Both have the lasted firmware. I switched to the spare for testing purposes.

Any add-on security or anti-malware apps, add-on VPN clients, add-on cleaner apps, or add-on firewall apps installed here?

None. I use Little Snitch but only in Silent Mode - Allow Connections for tracking only.

MrHoffman wrote:

....

The network is segmented. The ping responses show that. The question then becomes why, and what is on which segment.

I'm sorry, I think I've just confused myself with the multiple modem-routers that I was running for testing. There were actually two different networks, using the same brand, but different unit, of modem-router. So, the IP address was identical, but they weren't on the same network.

So, my ISP has sent me a third modem-router (I actually have four, including a mobile broadband modem). This one is a different brand and has a different IP address, so I'll get less confused. Okay, all setup and going and rebooted and the same error:

So, exactly the same behaviour. The new 27" iMac on Catalina boots up as if fully connected to the Ethernet network, but Network Utility can't find the IP address that Network preferences said that it found. Remove the Ethernet cable from the iMac and reinsert, and after a minute and a half or so, all is connected okay. I then connect a MacBookPro to exactly the same node, and it immediately connects.

So, just to recap, I can replicate this error on the 27" iMac, irrespective of what cable I use, or whether its DHCP or static, or what modem I use. However, I can't replicate it on a MacBookPro running Sierra on exactly the same cable and node. The MBP just boots straight onto the network on Ethernet. Unless there are any more suggestions, I think this is something for Apple and I'll tell you how I get on.

MrHoffman wrote:

.....

The network is segmented. The ping responses show that. The question then becomes why, and what is on which segment.

Here's the ping responses on the third and latest modem, for completeness:

And another, which is on the same network (my MBP), but which doesn't have any ethernet problems on book from the same node (though it's on a different cable for this test):

Okay. Some more testing. I've tested with another modem-router (Identical to the first). Exactly the same network behaviour - on boot, the Network Preferences panel says the Ethernet is connected with an allocated IP address , but the Network Utility says not, and indeed it's not. Removing and reinserting the Ethernet cable in the iMac achieves connection after a 1m25s wait.

So, it's not a faulty router. I'll test the cables next.

BDAqua, It is indeed!

A few more tests.

So, as I foreshadowed, I tested the Ethernet link with another Cat 6 Ethernet cable. Firstly, hot swapping would result in a connection, on a 169..... IP address - that is, no connection. I waited up to five minutes on three seperate attempts, but it never successfully acquired a real IP address (192.168....)

I then rebooted, and with the new cable, the behaviour was identical. On boot, Network preferences said that the Ethernet was connected with an IP Address, while Net Utility says the Ethernet link was Active but with a blank where the IP address would be, and there was no connectivity. Removing and replacing the Ethernet cable from the back of the iMac resulted in a 169... IP address within about 25 seconds which changed to a valid IP address (192.168....) at about 1m25s.

So, I also tested the same connection - same cable, same modem-router, same port in the router panel, with my late 2011 MacBookPro running Sierra. Hot swapping the ethernet cable resulted in a full connection with IP address within 2 or 3 seconds - too quick to measure the time. No problems with obtaining an IP address on boot.

I had a look at my router's network map, when connected, and all looks normal. I've had no IP Conflict errors and I can't see any candidates for conflict. Known connected devices each have their own IP once connected.

So, I think I've safely excluded cable and modem-router issues by showing the problem persists after changing both with this Mac and showing that I can't replicate the issue using the same cables and modem-router with a different Mac.

I'm beginning to think that there's something wrong with the new iMac's Broadcom 57766-A1 ethernet card or its PCI controller.

Or it could be a software issue with Catalina vs. Sierra, but I've not seen any reports of this.

BDAgua

So, I tried setting a Static IP in the router's range, starting after a reboot when I had the usual error: Network Preferences has a wired IP address, Network Utility doesn't.

Immediately, the Network Utility showed the static address and my router's net map in my browser, after a refresh, showed my iMac connected.

However I had no internet connection and pinging using Network Utility failed for everything beyond my router.

I then reboot, and got the same condition on reboot - that is, now my Network Preferences, Network Utility Info panel and my routers network map all agreed that my iMac had an IP address, but I had no internet.

Setting the connection away from Static (Manual) to DHCP didn't restore the connection and removing and replacing the cable didn't help this time.

However rebooting and then doing the remove and replace procedure with the ethernet cable after setting to DHCP did get me back to where I was. I'm not really happy though having to manually insert cables every time I boot to get an ethernet connection.

BTW there's nothing wrong with Wifi so I have an alternative. However, I just paid a lot of money for a new computer and I want it to work!

BDAqua wrote:

....

System Preferences>Network>choose interface>Advanced>Proxies Tab, make sure none are set, like for HTTP & HTTPS.

System Preferences>Network, unlock the lock if need be, highlight the Interface you use to connect to Internet, click on the advanced button, click on the DNS tab, click on the little plus icon, then add these numbers...

8.8.8.8

8.8.4.4

1.1.1.1

9.9.9.9

Apply.

No advanced proxies set. There's some bypass settings for 169.etc but they don't apply I assume.

I had the first two DNS settings that you suggest before I deleted all my Network settings. However, I've added those back in, plus the two others you suggested.

So, then tried connecting from a reboot and ....

Result is the same. On boot, my ethernet is connected with an allocated IP address in Network Preferences and Active with no IP address in the Network Utility Info tab. But not pinging anything - "No route to host" for 192.168.0.255