If I understand correctly, it sounds like you moved from an ISP that uses CGNAT to one that does not. Does your ISP provide a modem? If so, are you relying on the software features of that modem, or do you have a router inbetween?
this post was submitted on 16 Dec 2025
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You nailed the first part. As for the second one, you'll have to be patient with me, because I'm not tech savvy and english is not my first language. Mi ISP provides a router, and through the router settings, you can port forward. Is that what you are asking?
Yes, you got it.
It's possible that however your ISP provided router is designed, it's got some hidden port forward configuration. If that router has an option typically referred to as "bridge mode", you could bypass its routing features altogether and use your own router instead.
ISPs often have clauses about using their residential internet for hosting servers or exposed services, and it's possible your has taken a different approach to mitigating traffic from those sources.
If you can, I'd recommend using your own router rather than what the ISP provides.
I can't buy a router at the moment, things are not easy, and with the holidays coming, it's even harder. I had a look to see how much would I be in for and it's very much outside my budget.
Thank you nonetheless, maybe it's something I will be able to afford at some point in the future.
Are you sure nothing else is changing? How long did you do these tests for?
I'm completely sure! Yesterday it was running ok all day. Today, I port forwarded and left home for a couple hours, after coming home I checked the computer and saw the low speed. The first thing I did was think about what had changed and I closed the ports. The speed went up immediately. I tested it a couple more times with different ports and the same thing happened.
What the fuck?
Maybe they meter incoming speed? Try running a speedtest using a web server or iperf3 or something.
The speed test looks fine. Maybe they meter it as you say. Guess I'll have to contact them so they can explain wtf is happening to my connection.
How did you do the speed test? You need to have an open port on your side and another IP address outside your network.
Ah, I used the speedtest tool of cloudflare, ookla and openspeedtest. You are referring to https://iperf.fr/, right? When I get home I'll read how to do it properly!
I am a Unix person I just get it from my distro:s package repository. I don't know if that URL is the correct address.
The steps are (simplified):
- Get a server on your home IP (your seedbox will do) and a computer on the outside (you may prefer try a VPN) to stress test your connection.
- Open a port to the machine at your house. Run iperf3 server on that port on that machine.
- Connect to that machine from the one outside your home via a command which runs a speed test.
- See results.
This specifically speedtests incoming connections to your IP address. Regular speedtests like fast.com, etc. Test outgoing. With the way TCP/IP works, your ISP can easily differentiate the two.
Man, if this is the simplified version, I'm totally fucked. I understand every word, but the order in which they are stringed together confuses me.
I'm waiting for a call of my ISP's IT service to see what's what. If they can't (or won't) fix my issue, I'll bite the bullet and I'm gonna buy a subscription to a VPN.
Thank you for the detailed explanation!
Its identical ish to what you'd do if you host a website. You can also do that and just try downloading a large file from it (still while outside your network)
Would downloading a movie from jellyfin work?
Strictly, yes.
Dude, so sorry to reply to you this late. It's been a very chaotic week and I have not even thought about anything else but fucking work.
I'll try tomorrow, today I need to just be an amoeba.
Thank you for your help, and again, I'm sorry for leaving you hanging!
Oh you don't have to apologize.
What about other traffic?
Other traffic is fine. That's what was screwing with me, but after reading some people say they might be fucking with my connection due to seeing it is a bittorrent connection, everything makes more sense.
Are you running on port 6881? Pick a random one above 10000 and see if it changes.
No, I tried with a number of ports all on the 40k and 50k.
Just to be sure, did you already test that the port is actually open and forwarded? e.g. with your torrent client running browse to a port test website like https://canyouseeme.org/ , https://www.yougetsignal.com/tools/open-ports/ , etc. put in your torrent client's incoming port and check if the website can "see" your open port at your torrent client.
And the ISP (or router) itself isn't doing anything weird to block torrents, right? In your torrent client if you click any working public torrent, click on the Trackers tab, you should see DHT as working along with whatever open trackers are on the public torrent. In other words you won't see anything like "waiting" something (I forget the exact message you'll see when DHT is being blocked but it'll definitely not be working).
EDIT: Also if it's a new ISP with new router it might have firewall rules set up that are slowing things down, something to check.
Yes, I checked if they were open, and the three sites say they are. I used the ones you sent me, and same results!
Some pwople say that they might be throttling my connection, and I think it is what makes more sense. I'll have a call with them to see what is happening.
Thanks for pointing out the router firewall, didn't think about it, but iy is not that either.