We ask the root server for the us. name server.

Then we ask a.gtld.biz for the co.us. name server (notice we get the same answer–a.gtld.biz is authoritative for both us. and co.us. In other words, co.us. has not been delegated.)

We're using names rather than addresses for the server part of the command line. That will work if the server name can be resolved (DNS is mostly working). If not, you will have to use addresses which you can usually find using -v (verbose). Here is an example.