In an age of nat routers and dynamic addresses IP addresses are not suitable as identifiers. That doesn't mean you should have to sign up, though. It would be much better to use your existing login from another service via openid or something.

How about just using a cookie?

No, it's not going to keep track of me as I go from computer to computer, but if I care about that, I'm interested enough to login.

Oh please, give me a break. I don't want to have to log in at all. If ip address collisions become a problem THEN you can implement openid. Until then you will never get big enough to make it a problem.

Source IP as an identifier is a really bad idea. As an example, I work at a company with hundreds of thousands of employees. Do you know how many internet IP's we NAT them all behind? 2.

Also, while most ISP's don't assign customers new IP's frequently, they do do it occasionally. (At least, Comcast in Denver does). I finally signed up for dyndns because I got sick of not being able to access my home machine from work every 6 months or so.

OpenID/Facebook connect (like another poster suggested) would be a good choice, but source IP as a login identifier? That's just plain dumb.

are you missing the point entirely? i highly doubt that more than one person at your company will be using this service. i highly doubt that anyone at your company besides you will be using this service. this service probably has a total of 300 uniques, and maybe 5 active users including the creator.

Stack Overflow did essentially this, didn't they?

How did they do that?