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Root cause of Nighthawk C7800 Intermittently Dropping entire 5.0 or 2.4 GHz Wi-Fi Networks & A Fix

CryptoFox
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Root cause of Nighthawk C7800 Intermittently Dropping entire 5.0 or 2.4 GHz Wi-Fi Networks & A Fix

A LOT of people have been having this issue with the C7800; including myself! As far as I have seen, it is mostly people on Comcast, and I now know why! (If you know exactly what I am talking about, feel free to skip "Section 1: The Problem" section as it is just a description of the issue myself and other users have faced.)


Section 1: The Problem

After getting the C7800 up and running, it is AMAZING... At first.

After using it for a few months, many people, typically Comcast subscribers, start to have a problem with one the two WiFi frequencies. For me, and (based on anecdotal evidence) most everyone with this issue, it is typically just the 5.0 GHz band of Wifi.

That said, I have seen that some people experience this on the 2.4 GHz band, too. But it is unusual for both bands to drop at the same time.

Often, people are advised to check the range and see if there is interference from other WiFi networks. But it doesn't help.

The problem is that the entire frequency just stops broadcasting. If the frequencies use two different SSIDs, then you will notice that you cannot see the SSID of the dropped frequency on ANY device, no matter how close you get to the antenna. Further, all devices that were connected to that frequency lose connection at the EXACT same time.

This issue usually takes a few months to crop up, making it hard to get tech support for it or to return the C7800. Those that manage to get a replacement will be happy that the issue goes away immediately, but it usually comes back after a few months.

Well, I paid for Netgear's Gearhead support (so you don't have to!) and I FINALLY got the real answer to the problem! I now know the root cause of this issue!


Section 2: The Cause

I hate to break the news to you, but your modem/router is experiencing a DoS attack; at least it believes that it is. If you to the Advanced tab and navigate to Adminstration --> Logs, you will more than likely see a few entries labeled with "DoS attack".

If you use the CTRL-F keyboard shortcut and type in "DoS" you can quickly find the entries, and see how many there are.

Note, if have recently done a factory reset on your C7800, there will be fewer of these because doing so erases all log entries from before the reset.

Some of you who already know what a DoS attack is might be asking why only 5.0 GHz or the 2.4 GHz is affected if your C7800 is being DoSed, shouldn't it affect all devices trying to connect to the internet? And why would it stop you from seeing the Wi-Fi SSID? Shouldn't you still be able to see your internal network even if you can't reach the internet?

I had the same questions. But here is what I was told by the Gearhead staff:

The router/modem has an inbuilt security function to protect your internal network in case the pre-programmed firewall senses a threat.

It disconnects portions of your internal network to try and limit traffic as well as to protect those devices. It maintains service for anything plugged in with an Ethernet cable so that the Router/Modem can be administered and you, or a technician can try to fix the problem.

It may only shut down one WiFi frequency so that some devices can still connect wirelessly for the same reason it maintains connection through Ethernet.


Section 3: The Solution(s)

A VERY temporary solution is to press the Wi-Fi button on your C7800 to turn of the wireless signal. Then, press the button again to turn it back on. Both frequencies should come back on, but if you are still being DoSed, it may immediately experience the problem again. The same goes for unplugging and plugging back in or pressing the power button to turn off and on.

The best solution is to get a new public IP address. This is because the public IP address is what is used to target you in the DoD.

Most ISP's assign IP addresses to users dynamically, and they frequently change so this problem doesn't ever get to sever. In fact, for these lucky people, the problem probably goes away on its own after a while and they won't know the difference.

If you don't have Comcast, and the problem doesn't quickly go away, you can usually just call your internet provider and ask for a new set of IP addresses and that is that. If it comes back, ask them to shorten the lease time of your IP address and it will be unlikely for the problem to ever come back.

Those of us unfortunate enough to be stuck with Comcast will have a MUCH harder time; I hope you have another compatible router/modem handy, because you will need it (unless you switch to a new ISP).

So here is the rub with Comcast. They dynamically assign IP addresses to NEW routers on the network. If it recognizes the router's MAC address (the hardware address built into your router) then it will just give you back the same exact IP address pretty much every time you connect.

However, because there system is dynamic and all their support is pretty much automated, you will not be able to just call and ask them for a new IP address.

Either, they will tell you they are assigning a new one, but it will stay the exact same, or tell you to power your router off for a period of time, then power it back on for a new IP address, but it will not change, they will tell you to contact the hardware manufacturer (Netgear) to get help changing it, tell you it is impossible for them to change it, and/or say they need to send a technician to your premises to change it.

Something about their setup or their training makes it pretty much impossible for you to get them to manually change it on their end, and the way they maintain the IP addresses for so long after originally assigning them means that most people are stuck with the same IP address for 2-3 years.

So, how do you FORCE Comcast to give you a new IP address? Well, with the C7800, the most viable way to do this is to just leave your C7800 off for the duration of the IP lease. You can check this duration by going to the Advanced tab then clicking the "Details" button on the right hand side of the main page. A separate window should pop up with two buttons on the bottom: "Release" and "Renew". There will be information on how much longer the lease lasts in there.

For me, it is usually between 3-4 days, so that would mean 3-4 days with no internet. YIKES!

I haven't tried it, but one option may be to click the Release button (without clicking the Renew button), then powering down the C7800 for 2-3 hours in hopes of getting a different address. But I doubt that would work.

You could also try turning the C7800 off a few hours before the lease expires and turning it back on a few hours after. But it might not do the trick.

Alternatively, you can buy a second C7800 and switch back and forth between them whenever you start experiencing this issue. It should buy you enough time for Comcast to forget the MAC of the first one.

Some people may know about MAC cloning on routers and may be thinking they can just change the C7800's settings to get a new MAC address and, thus, force Comcast to give them a new IP address.

Unfortunately, you cannot do this because of the fact that the C7800 is a combination router/modem. If it was only a router, it would be possible. However, it cannot be done on a modem because a modem's MAC address is super fundamental to the way cable internet works.

My advice? Ditch Comcast for another cable provider that works with the C7800. If you can't do that, buy one router and one modem as two separate devices so you can get a new IP address whenever you want.
Model: C7800|Nighthawk X4S-AC3200 WiFi Cable Modem Router
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