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how does host B know that its visitor is the one that visited host A?

From: Jonathan Rees <jar@creativecommons.org>
Date: Fri, 12 Aug 2011 11:11:48 -0400
Message-ID: <CACHXnarZb2z+GTOZB1WPLC-Dh6s_U2Q=2czzWCvQy1Fhn86M+Q@mail.gmail.com>
To: www-tag@w3.org
Probably everyone knows this but me...

I shop at expedia.com (or somewhere) for a London hotel room. Later I
visit guardian.co.uk and see an Expedia ad for London hotel rooms.

I visit guardian.co.uk in a different browser (same computer & IP
address but Safari instead of Chrome) and instead get an ad for
magazine subscriptions. Apparently the Guardian can tell my two
browsers apart somehow - it's using more than just my IP address to
decide what ads to show me.

How does this work? I.e. what are browser instances doing that leaks
their identity to servers? Is it just a lucky guess based on
User-agent or something?

(a propos our privacy & tracking discussions)

Thanks
Jonathan
Received on Friday, 12 August 2011 15:12:23 GMT

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