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I've always asked this question in my mind but couldn't answer it. I searched about it and found only one search result ( https://community.spiceworks.com/topic/338720-how-do-isp-s-monitor-your-data-usage ).

I know not all the ISPs use one constant way. But any thoughts about some ways of how does an ISP know the bandwidth usage?

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    Not sure if I understand the question. All your traffic goes through the ISP's network, so they can count it. The answer you linked to explains this already. What specifically do you want to know? – slhck Nov 21 '19 at 12:52
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    Everything goes through the ISP's network so they can surely count your quota. Just like on phones cellular data with limited quota, once you reach the quota you can't use it anymore. – CaldeiraG Nov 21 '19 at 12:59
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    Even your own computer counts the number of bytes sent and received and any carrier-grade switch would as well. It's easy to lift those numbers out of the switch using a script or tool. The hard part is to do it on a carrier-grade scale, as it would involve some computing for every single port on the network (potentially millions, one for each subscriber). However, carrier-grade equipment has all these functions built-in. – mtak Nov 21 '19 at 13:45
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TL;DR you put data through your ISP, ergo they know exactly how much data you put through them.

Even if data is encrypted and/or goes to a VPN the actual packets have a minimum of three pieces of information that can never be encrypted in order for the data to be transmitted at all.

Those three pieces are

  1. Source address
  2. Destination address
  3. Packet size

(If you run a VPN the packets will contain encrypted packets for the VPN to decrypt and pass on but the external packet is still a packet itself, your ISP will know that you are sending data to the VPN IP.)

When you send any data through your ISP they will be able to collect aggregate data of what came from your current IP and how much of it there was. There may be far too much data for them to actively store it all, but that does not mean that they cannot keep a running total of packets and their sizes.

It may even be mandated by local laws that they actively log source, destination and amount of data transferred as well as any other metadata they have available. YMMV. IANAL.

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