Sharing an internet connection

Suppose one house has a high-speed internet connection with a wireless router. Further suppose that a neighbor can connect to this router and (unlike in many past discussions) the subscriber knows about and has no problems with such shared usage.

Is this sharing detectable by the internet service provider? Is it typically condoned by an ISP?

I can’t see how they would be able to distinguish this from simply another user in the same house.

There’s no way an ISP can differentiate a wireless connection within a home from one between homes.

Is there any way for an ISP to determine how many computers are in use? Or the ID (MAC address?) of devices that are using an internet connection?

I suspect that the router’s MAC address is as far as they can see - given a properly secured router.

I really don’t know jack about the protocols involved, but wouldn’t the packets your router handles need to have an envelope that identifies which local address they are coming from/going to? When you click on a link, the HTTP GET originates from a single machine on your home network, and the response needs to find its way back to the same single machine, so it must contain identifying information of the local machine.


Actually, the information on where the response goes is not in the packet, but is kept in a table in memory in the router while the connection is open. From the web server’s point of view, the request is coming from your router and goes back to your router, and when the router receives the response, it looks in the table to figure out what local computer to send it to.

However, it’s not completely true that the ISP can’t figure out how many machines are behind your router sharing your connection. While, technically, that information is hidden, they’ve come up with techniques that allow them to infer it. For example, each TCP packet that a computer sends out has a sequence number. The sequence number increments in a predictable way. So if they see sequence numbers like this:


They can infer that there are at least three different computers generating packets behind this router.

I think ISPs might have given up on trying to restrict the number of computers using a given connection, however. It’s too hard to write and enforce meaningful rules, and it’s much easier to simply throttle the bandwidth…

We always have at least three computer online, a couple of them wirelessly. When my next door neighbour can’t get a connection, he hooks in to ours. Sometime I use his, just because I can. We are both cool with it. There are often more computers in either house online too, which would be a major headache for the ISP’s (Not the same ISP) but they have never reacted - yet!

No. Most ISPs don’t allow it. In fact, in my service agreement I agree specifically not to do this. I also agree not to run a server from my home, which is another typical ISP requirement.

You’re not likely to ever get caught if your neighbor piggybacks off of your service, but technically it is a violation of your service agreement for most ISPs.

Most ISPs have no problem with you running multiple computers in your home. It’s just when you get your neighbor’s home involved that they have an issue with it.

I haven’t figured out the effective protocols yet (it isn’t easy and the ISPs and manufacturers don’t seem anxious to explain!), but I believe it depends on the kind of connection. If you are using broadband cable, the router seems to be the only address readily available to the internet. If you are using DSL, you are likely using PPPoE, which if I understand it correctly, sends the MAC address of every connected computer out to the ISP. Further if you use your router in “bridge mode”, you are clearly doing this. [MAC addresses, for those of us who don’t do this every day, have nothing to do with an Apple Macintosh. It is the unique serial number of an ethernet card. Every ethernet port ever made has a unique address. Each manufacturer gets is assigned a number and adds a unique serial number to their number. This is necessary on your LAN so that each computer can keep pick out ethernet packets intended for that machine. It has nothing to do with the IP address. It is unique to the computer you are using and unique to the ethernet. Over networks use different IDs]. If you are stubborn (took me weeks to do on my own), you can set up a DSL connection that does not limit the number of computers attached to the network. But I couldn’t do it as long as I used any of the hardware provided by the phone company. And if you don’t use their hardware, they won’t help at all.

Also, it depends on the software you use. Most (all?) email packages send the local IP address with each post. Of course that can change at any time, but in practice most people change their local IP address rarely if ever.

In sum, maintaining your privacy on the internet ranges from difficult but not impossible.