IP:

Some Internet services use the source address of the client’s computer as a form of authentication. These systems keep track of the Internet Protocol (IP) address that an end user used the last time that user accessed the site and try to determine if the user is legitimate. When that same user accesses the site from a different source IP address, the site asks for further authentication to revalidate the client’s computer. The theory is that a user’s typical location computer has a somewhat persistent IP address, but when the user has a new address, that user may be mobile or using a less secure wireless media, and then require further authentication.