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I completed a survey that stated I am only allowed to do it once per day. If I try to do it again, I am not allowed as expected.

I am trying to find out how they know that it is from the same computer/network.

I tried doing the survey with another computer on the same LAN but it still didn't work. Then I thought they might be using my public IP and after changing that, it still didn't work on either computer.

Now I'm wondering how they uniquely identify my local network without relying on my public IP address? My next guess is that they are using the MAC-address of my router.

I want to know what are common/popular methods that websites use to do this type of thing.

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There are lots of ways to do this. Its not clear how you changed your assigned public address, that is something, most people don't have control over. Of course what you describe could have been simply done by a cookie. Your MAC Address is not broadcast to websites. –  Ramhound Sep 6 '13 at 12:16
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4 Answers

Websites cannot have access to your Mac Address through your browser.

Using the public IP is most likely the case, since they didn't let you take it again on another computer.

After changing your public IP, you mentioned that they still didn't allow you to take the test, thus they are most likely saving a cookie on your browser which says that the browser can't take the survey till the next day.

So if you want to really check, change your public IP AND clear your cookies (Or just open a private/incognito tab in your browser, or use another browser). Only after you have done both actions, check and see if you take the survey again.

Of course, another way would be if you are signed in the website you are using, then they would it isn't something you can do, except creating another account.

Disclaimer:
This answer is for informative purpose only. If the website where the survey you are taking doesn't allow you to take another survey in the same day (specially if it's some form of paid services), it's most likely against their terms and conditions.

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"Then I thought they might be using my public IP and after changing that, it still didn't work on either computer." I have tried changing my public IP and clearing my history/cookies and using incognito all together and it still doesn't work. I'm just curious as to how a website can identify my LAN. (Everyone is starting to worry about the survey and it being against the site's rules... I am not trying to exploit anything or earn money. I'm simply curious) –  Piet Sep 6 '13 at 12:20
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One possibility is your ISP. Your network's IP address may be being mapped to an ISP's address before being passed to the surveyor's servers. If this is the case then if the lease on the mapped IP address hasn't expired, changing your local network's address won't have any effect. The ISP will still see your local network as connecting on a specific ADSL line.

Not all ISPs do this and where it is done it is to try to re-use IPv4 addresses within the ISP's customer base.

(...and do you really need to do a survey more than once a day - against their rules?)

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They are most likely doing checks for BOTH IP address AND A cookie (or some other characteristic served up by your browser). As stated by someone else, it is almost impossible for them to be using your MAC address - for 2 reasons - 1 - Your MAC address would change when you change computer and 2, your MAC address is not broadcast onto the Internet.

It might also be that when you changed your IP address you were changing an Internal IP address behind your router, and the router rewrote it to its external one - this is common practice. If your IP address started 10. something, 192.168.something or 172.[16-31].something this is most likely what happened.

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Ways to uniquely identify a computer remotely apart from IP address (or MAC, which isn't possible):

  • HTTP cookies
  • HTTP DOM storage
  • If you have an HTTP proxy such as Squid, sometimes these proxies can pass your private IP in a Via: HTTP header
  • A Java applet or Internet Explorer ActiveX control can get many of your system details. You'd have to have allowed the plugin to run previously.
  • I have a faint memory of reading something about a way via Javascript that determines the fonts on your system, possibly causes another one to be installed, and checks for that font.
  • Flash cookies
  • Adobe's Flash engine may also have access to many system details.
  • Other browser plugins or extensions can cause your system to reveal information about itself to the system you're trying to connect to.
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