session tracking

So I'm making my first PHP website and it's just so deliciously cool I can't even describe how I feel. But that's not the subject of this post.

I've been taking notice of how different websites handle sessions and it seems like most of them use cookies. I was going to just use IP addresses. I know IPs aren't foolproof since ISPs re-use IPs for different people, but for a small, not too serious site, I don't see the problem with it.

But seeing how many use cookies has made me nervous. What do you all think?