Quote:
Originally Posted by pseudonym
A user might fo a fair amount of time and do quite an amount of browsing without visiting a site carrying a Nebuad advert - I guess you could track a user if he follows link to other sites by checking for the referal header, but once he enters a URL in address bar you'd no longer be certain if it was the same user, or another user at the same address sharing the connection, or even if the IP address has been freed-up and allocated to another user.
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I managed to attribute the previous quote to the wrong New York Times blog entry about NebuAd. It should have been:
NebuAd Observes ‘Useful, but Innocuous’ Web Browsing
Mr. Dykes said that the company also examines other information about users’ computers in order to identify when an I.P. address is changed. But he declined to explain what that information is and how it is used. 
Dykes also goes on about the benefits of the vagueness inherent in using only IP addresses. That article is dated early in April. It looks like things have moved on. I remember being puzzled by:
Infighting At ISPs Over Using NebuAD
I’m told NebuAD is even able to build profiles of individual people using the same IP address (ex: users behind a NAT device).
While it may have once been an IP address tracker, it certainly looks like a cookie tracker now. As you say, it’s not how often you can read the cookie, but
when you read it. You only have to read the cookie when there is no referrer header. You can link up all the subsequent pages, with a high degree of reliability, using only the IP address and later referrer headers. You’ll have a root page and a fairly sparse tree.
I imagine only a handful of such root pages cover a very large proportion of all web browsing graphs.