Forum OpenACS Q&A: Response to Serving up OpenACS (private IP addressing)
The IP ranges that have been set aside for non-public networks are 192.168.xxx.xxx , and 172.16.xxx.xxx, and 10.xxx.xxx.xxx . It is always safe to use IPs in this range on your local network; especially since any packet addressed to either network will be dropped at the first router, making some kinds of network attacks impossible. You can learn more by reading RFC 1918.
While it won't bother anyone if you use IP addresses other that this range (since your ISP will not be routing packets to you anyways) it could cause you to not be able to access some sites, since your router will be looking on the local network and not on the public Internet for a machine with that address. It is considered bad practice to do so however.