C:\Documents and Settings\Anton Daneyko>nslookup superuser.com
will look up your own DNS server to find out the IP address for superuser.com. If you add the ip address or the name of a different DNS server to the command line, it will lookup that given DNS server for the ip address of superuser.com. Ex:
C:\Documents and Settings\Anton Daneyko>nslookup superuser.com 188.8.131.52
By the way, 184.108.40.206 is the ip address of Google DNS servers.
But, both of the above give "Non-authoritative answers", as neither of them SOA, which is the authoritative for superuser.com domain. Both have a cached copy that has been propagated from the SOA. If you want to ask the authoritative server, first find out the name of ip address of the authoritative server, using the command:
C:\Documents and Settings\Anton Daneyko>nslookup -type=ns superuser.com
superuser.com nameserver = cf-dns02.superuser.com
superuser.com nameserver = cf-dns01.superuser.com
cf-dns02.superuser.com internet address = 220.127.116.11
cf-dns02.superuser.com AAAA IPv6 address = 2400:cb00:2049:1::adf5:3b04
cf-dns01.superuser.com AAAA IPv6 address = 2400:cb00:2049:1::adf5:3a35
cf-dns01.superuser.com internet address = 18.104.22.168
This will return a non-authoritative answer from your local DNS server, from the Marburg Uni, naming all the authoritative servers for the superuser.com. Then you can use the command we used earlier to ask any of the 4 authoritative servers, as follows:
C:\Documents and Settings\Anton Daneyko>nslookup superuser.com 22.214.171.124
As you see, this time the authoritative SOA server returned the ip addresses, hence you don't see the comment "Non-authoritative answer" comment, anymore.
This is particularly useful, when you have created a new domain name or changed the hosting providers or transferred to a different domain registrar, and you can't access your website, as the new IP addresses haven't propagated even after 24 hours. Then you can start with the SOA and verify that your correct ip address is given by the DNS server, and then follow it further down the tree. Good to check if Google DNS servers have received the changes, and then lastly if your local DNS server can resolve your Domain name to correct IP address.