Although what you say is common it is not true for all routers. E.g one router I used to use defaulted to 192.168.0.2 and 192.168.0.3 for the DNS servers. And I have seen some that give the real DNS servers of the ISP. So it is essentially what your DHCP server assigns. And for the person who said he had specific ones he needed to use, most Routers allow you to specify the dns addresses assigned by there builtin DHCP server.