In addition to Richard post, each IP has something called a TTL (time to live). This is an amount of time a NIC may hold onto a IP address before it has had to hand it back to the DCHP server for reallocation and then has to renew it's IP address with the DCHP server again. This is probably what was happening with your PC and for some reason that exchange didn't work as it should, hence the connectivity problems and the warning.