Ever have a real "Homer Simpson" moment?
I was installing a network card in a pc and whenever I tried to enter the IP address and subnet mask I kept getting a message that "The specified IP address is not valid…" I tried reinstalling, updated drivers, even a different card. Even wiped the hard disk and reinstalled windows. Eventually went back to the knowledge base and looked up the error message. MS suggested editting the registry to enter the IP address.
Gave it a shot, and then saw I was typing the IP address in the subnet mask box and vice-versa. 🙄
So, in conclusion, "D'oh"