Help: Xfinity Internet slow speeds?

100Mbit/s is what I got on my iMac using Network Utility

I would like for you to do the following:

1. Power-down the Netgear gateway, the AirPort base stations, and your iMac. Order doesn't matter.
2. Connect a single Ethernet cable between the Netgear and the iMac.
3. Power-up the Netgear. Give it about 5 minutes to initialize.
4. Power-up your iMac. Leave the AirPort base stations powered off.
5. Using the Network Utility, what is the Link Speed read now? Is it still 100 Mbit/s or 1 Gbit/s?
6. If it is still 100 Mbit/s, either the Netgear, the cable, or the iMac's Ethernet settings is faulty/wrong.
7. If it is 1 Gbit/s, then power-down the Netgear and iMac. Again order doesn't matter.
8. Now connect your "main" AirPort base station to the Netgear, using the Ethernet cable that was connected to the iMac. Connect a second Ethernet cable between one of the available LAN (opposing arrows) ports on the base station to your iMac.
9. Power-up the Netgear, wait at least 5 minutes for it to initialize.
10. Power-up the AirPort base station, wait until its status LED is a solid green.
11. Power-up the iMac.
12. Again, use the Network Utility. What is the Link Speed now?
13. Did it drop back down to 100 Mbit/s or is it now 1 Gbit/s?

On the iMac .. which I gather is in the same room as the Extreme that is working as router.. try using wireless instead of ethernet and give us a speed.

If that gives you high speed..

I would disconnect the Airport working as main router and plug the computer directly into the modem.. you will most likely need to power cycle the modem.. off for 10-15min is enough but can be longer.. and then get the speed.

Exactly what model of modem are you using..

Something does not make sense because if wireless on iphone etc is so fast.. I see no issue between the modem and the airport.. yet all your system appears to be slowed down once ethernet is involved.

Another test that is worth performing since you have three airports.. swap one of upstairs one with the router.. reset it to factory and use a different configuration name etc.. basic as possible.. and see how it goes.

Shouldn't my speeds be higher than 90s I'm getting?

Ethernet that is damaged or incorrectly installed or cables made up badly will either not work at all which would be easy to detect.. or they work at 100Mbps.. which would give your 90Mbps speed.

Tesserax is asking the right question. Somewhere you have a badly installed cable.

Power off the CM 600 modem completely by pulling the power supply cord from the back of the device, or pulling the power supply from the AC wall jack.

Leave the modem powered off for at least 30 minutes....60 would be better if possible.

After the modem has been powered off for at least 30 minutes, connect the Ethernet cable from the modem to the iMac

Shut down the iMac

Power up the modem and let it run for 5 minutes by itself

Power up the Mac

Make sure that WiFi is turned off on the Mac

Check to see if the Mac now has a good Internet connection. If yes, run a few speed checks.

Note....you will need to follow the same type of procedure when you later connect the modem to the AirPort Extreme.

When you test.. do not plug the upstairs into the Extreme at first..

Just use modem to wan port of the AE.. and Mac to LAN port of AE.

As Tesserax indicated already ensure the connection is gigabit.

You can also use network utility which Apple has now hidden.. just use search in finder.

Click on info and you will see the connection speed.

If the port for the modem or iMac was faulty, then shouldn’t my internet be down altogether?

If wireless is on it should still work fine to connect to the internet.

That is why I asked you to get another computer with gigabit port.. or buy the thunderbolt to ethernet adapter for your Mac.. the right one for the model of course. Don't buy USB type as they are usually 100mbit.

Disconnect the Ethernet cable from the modem

Follow the instructions provided by Netgear to reset the modem.

Once that is done, power off the modem for a few minutes.

Then, connect the Ethernet cable from the modem to the Mac

Power up the modem and let it run for a few minutes by itself

Power up the Mac and see if it can get an Internet connection using Ethernet only

Things will either work or they won't. If they don't, then there are two possibilities:

1) The Ethernet port on the Mac is not working correctly. Try another computer to see if can connect that way.

2) The modem is not resetting correctly, and you will need to get in touch with Negear support to likely replace the modem

Two things from your image:

1. The iMac is not getting an IPv4 address from the "modem." It should get one whether it is performing as a router or as a simple modem. This could be that the "modem" is incorrectly configured or that you did not allow sufficient time to allow it to "forget" the previous device (the Extreme) that was connected to it.
2. I've never seen Apple use NVIDIA chipsets for their Ethernet port. I would have expected that it would be some variation of a Broadcom chip. Has this iMac ever been repaired? ... and if so, was it done by Apple or an Apple Authorized Service Provider or by some other third-party repair facility?

May be hopeless.

If you have not already done so, try Netgear's instructions to reset the modem.

At this point there should only be the Netgear and the iMac connected by Ethernet. If you are getting zero for the link speed, the only possibilities are:

1. The Ethernet port on the Netgear is faulty,
2. The cable is faulty, or
3. The Ethernet port on your iMac is faulty.

Sorry, you have lost me.

We were talking about connecting the iMac directly to the modem to make sure that things are working that way, so you can get a real world reading on what your actual Internet connection speed might be.

So far, everything that you are telling us indicates that the modem is not resetting correctly.

Did you follow the Netgear instructions to reset the modem and then connect the Mac directly to the modem?

The bottom line.....no progress will be made until you are able to connect the Mac directly to the modem using Ethernet to make sure that the modem is working correctly, so you can get an accurate reading on the speed that the modem is providing.