Assigning DNS

[FdePaula]

Hi Guys,
I want to set Opendns DNS in my Private network.
When I open my network adaptor configuration, I see 2 networks, : One is local connection, from my ISP, and other is Ethernet.
I ask you if I am supposed to inform primary and secondary DNS addresses in both networks or only in first one (my ISP)?

TIA for any help...