DNS server addresses are assigned automatically by DHCP from your ISP.
If you are connecting to the internet via a router, the router will get the addresses allocated by the ISP. Depending on the router, it may then pass these addresses on to the "client" PCs or it may allocate its own IP address to the client PC's as their DNS server address.
You can also allocate fixed DNS Server addresses in the TCP/IP settings of your PC's internet connection. If you want to do this, try: