You should ask the Network Admin for the IP range since you have permission to use the wifi, right?
if you do not have permission, you should know that in many countries (Including the USA) it is illegal to connect to networks which are not yours. Its a invasion of privacy. Just because some people are not smart enough to secure their access points , does not mean you are allowed to use it. I alway compare it to this situation. If you forget to lock the door of your house do you think it would be cool if I came in and had a few of your beers and piggybacked your wife?