Is IP address unique for each computer?

[impact]

Hello,

I am in dilemma.

In my site, although i get very limited visitors, i have a basic short url maker which I did for fun. Recently, i came across bit.ly and i must say I am impressed with the fact that, all short url i created are actually visible to me, without even login in or having an account in bit.ly .

It is pretty obvious that, they are using cookies to store some sort of unique id of mine, with which they are able to get my data from the database.

So now I am in dilemma, as to what i should use to uniquely identify each visitor.

Any help will be appreciated.
Thank you,

[FourDegreez]

You can't rely on IP addresses, because oftentimes they will be dynamically assigned by the ISP. You can generate a random string and use it as a key associated with user data. Set this key in a cookie, and on subsequent visits you can read this cookie and use the key to look up the data. Of course if a user clears their cookies, you've lost the connection to their data. There is no way around that except giving users a login/password.

[KenB]

Another problem with relying on IP address is that on some corporate, academic and other private networks as well as some ISPs all computers are put behind a single IP address using a NAT. This means there could be dozens, hundreds or in extreme cases thousands of computers sharing the same IP address.

[impact]

Thank you all. I appreciate your help.