I am guessing you have an Airport that you connected to your ISP's router. This is producing a double NAT. If this is the case, simple change your Airport from Router mode to Bridge mode. This will turn off the DHCP service on the Airport and allow it to bridge the primary network. All devices will be on the same subnet.
Reid
Apple Consultants Network
Author "Yosemite Server – Foundation Services" :: Exclusively available in Apple's iBooks Store
Author "Mavericks Server – Foundation Services" :: Exclusively available in Apple's iBooks Store
Author "Mavericks Server – Control and Collaboration" :: Exclusively available in Apple's iBooks Store