Do you have a second ethernet interface on the server? Perhaps the other IPs are suppose to be running through that Ethernet interface?...
Are the additional IPs using the same gateway? Strange that they would be completely different Class A IP addresses that use the same gateway....
Are you sure? What web server are you using? What specifically is set for the DocumentRoot in the webserver's configuration for this subdomain?
What is the DocumentRoot for the subdomain?
Separate names with a comma.