A valid IP address must be in the form of xxx.xxx.xxx.xxx, where xxx is a number from 0-255. There are a few reserved addresses (RFC 1918) that cannot be used. 10.x.x.x cannot be used along with 192.168.x.x, 172.16.0.0 to 172.31.255.255. If you are using a router, your machine will likely have one of these, and you should see this tip for more details. If you have an IP address in this format, but do not have a router, your ISP may be using a NAT network, which will prevent you from running a server.