How is it that a ethernet cable connects with MacBook but not iMac

Hi HWtech,

Thanks for your message.

The set up is followed:

1) Computer in a room -> 2) ethernet cable (cat 6) -> 3) house ethernet jack (cat 5e) -> 4) switch in the basement via hidden ethernet cables inside the house -> 5) router (provided by internet service provider) to another room via hidden ethernet cables inside the house -> 6) modem (internet service provider) in another room via hidden ethernet cables inside the house

I have checked all the connections in reverse order from 6) to 2) using my laptop. I am getting the internet speed I expected at each site. The network perferences show a valid IP address. So I am getting the expected signal from 6) to 2).

When I substitute 1) with my desktop instead of my laptop, it doesn't work. But when I connect my desktop to the router in 5), it works. So my desktop is recognizing my modem.

I don't think it is related to SMC reset and Pram reset on my iMac. I have tried and it makes no difference.

I am connecting with DHCP.

I don't think it is relating to any firmware update, because the desktop is recognizing the modem. It is know when I put the desktop in location 1) that it doesn't work.

The ethernet jack 3) was a modify jack (Originally a phone line but was modified by a professional to an internet line/cat 5 jack). It was working with another old iMac before and my current old MacBook Pro via ethernet. So I was getting a solid connection and I am still getting a solid connection if I connect with my MacBook Pro. Could this be the reason why the new iMac doesn't work?

I thought about it too. However, I am careful to make sure there are no other cables running in the vicinity. Besides, if the iMac and the MacBook Pro are on the same desk right beside each other, it shouldn't explain why it works on the old MacBook Pro and not on the new iMac.

I am using speedtest.net to test to make sure I am getting the speed I've expected.

Maybe that's the explanation why the new iMac desktop doesn't work. I just tried it with a brand new PC desktop and it also doesn't work with the Ethernet cable.

Thanks for all your reply.

But if you look at the diagram, signals goes from 6) to 1). So if I am getting good signal and consistent download speed from 5) to 6) using my laptop, then that means everything upstream should be working fine. That includes the switch (which is brand new and I have tested it to make sure it works), ethernet jack (3).

For the switch, i have tested it by connecting the new desktop to the switch and then to the modem and it works. It's the most updated dlink model. But the desktop doesn't work when I put it in another room, downstream (following my diagram above).

Again, the problem is that at 1), my old MacBook pro works and not the new desktop. This is odd considering that newer computers should be more "versatile". I have both computer on the same desk.. it works on the laptop but not on the desktop. Weird...

My new desktop is getting a valid address when it is connected to 5. So it is picking up the signal.

I know what you mean. Sometimes if you prioritize the ethernet connection it should work. I did try turning the wifi off. I doubt the order matter in this case because the iMac does connect with the router in 5). I would imagine the signal sent off downstream should be the same... assuming that the download and upload speed is unchanged.

Thanks anyway.