I had a successful pairing between my iMac and my PS3 which were both hard wired to the same router (Telus Actiontec V1000H). These were from different ethernet wall jacks in the house on 2 different floors. The Actiontec router is located on the basement floor and was giving a poor wifi signal for other devices so I hooked up a D-link (DIR-625) to the main floor ethernet and connected the iMac (media file computer) to the LAN out of the D-link. Since this time my PS3 can no longer find the media server. I have forced the IP address of the Mac on General Configuration page of the PS3 media server main panel. I have turned off all firewalls and enabled all UPnP (PS3 says UPnP is enabled). I suspect its something to do with the following quote however this is where it gets a little over my head. If anyone could offer laymen help that would be so helpful!!
Your server and device are on different subnets:
Per the DLNA protocol (which uses UDP broadcast packets), the server and device must be on the same subnet. The easiest way to check is if you can ping the device from the server (successful ping = on same subnet). This issue should only really happen if you have multiple routers or managed switches in your network (in which case you'll need to bridge your routers together). If you only have a single router/switch, then you incorrectly set a manual IP address on at least one of your devices leading to a subnet mismatch