An unreliable technique for tracking users is based on storing the IP addresses of the computers requesting the pages. This technique has been available since the introduction of the World Wide Web, as downloading pages requires the server holding them to know the IP address of the computer running the browser or the proxy, if any is used. The server can log this information whether or not cookies are used.

However, these addresses are typically less reliable in identifying a user than cookies because computers and proxies may be shared by several users, and the same computer may be assigned different IP addresses in different work sessions (as is often the case for dial-up connections). The reliability of this technique can be improved by using another feature of the HTTP protocol: when a browser requests a page because the user has followed a link, the request that is sent to the server contains the URL of the page where the link is located. If the server stores these URLs, the path of pages viewed by the user can be tracked more precisely. However, these traces are less reliable than the ones provided by cookies, as several users may access the same page from the same computer, NAT router, or proxy and then follow two different links. Moreover, this technique allows only for tracking and cannot replace cookies in their other uses.

Tracking by IP address can be impossible with some systems that are used to retain Internet anonymity, such as Tor. With such systems, not only could one browser carry multiple addresses throughout a session but also multiple users could appear to be coming from the same IP address, thus making IP address use for tracking wholly unreliable.

Some major ISPs, including AOL, route all web traffic through a small number of proxies which makes this scheme particularly unworkable.

However some things make IP addresses more usable. With the much more widespread use of routers many broadband users retain the same IP for as long as the power stays on. Also combined with the User-Agent HTTP header, a tracker can differentiate users behind a NAT. When combined with session cookies and links to their tracker on huge number of pages (as happens with Google for example with Adsense, and Doubleclick combined constant IP announcing pings to their GMail service; among other providers and advertisers.) a tracker can track extensively even in the absence of persistent cookies.

IP address
Privacy and third-party cookies
Cross-site cooking
Cookie attributes