Due to heavy snowfall I was recently stranded on an airport. Having to do some surfing on my laptop I found an open access point. It offered 30 minutes of free surfing a day. I registered and used up my time.
Then I wanted to see if I could use it again - mostly just for the fun of it. I opened a different browser then before and tried to register again.
It didn't work. The access point recognized me and told me that I'd have to wait another day to get 30 free minutes again. I reconnected again to force a new IP - still the same.
How did it recognize me?