Windows 7: How to renew lease or renew ip address on the computer?

Hi
i have a slight problem with my pc, whenever i am on my public servers wifi, it always says that it is an "unidentified network", i have used this network before and this is a reoccuring thing, however my home router is completely fine and i can get internet. When i looked at my friends laptop i noticed that he has a lease for the network that i think that he has to renew or his pc does it for him and he has ipv6, so when i look at mine why doesnt it have it, (under network and sharing). However when i renewed my ip adress on another public network with internet, i then reconnected to the original public server wifi, i got internet connection, why is this that i have internet when i renew the ip and i had to do it manually and why does it show "unidentified network" when i had this internet connection before.
Thank you for replying