Dns

CNAME and A DNS entries

An A (Address) record maps a domain name to an IP address. A CNAME (Canonical Name) record maps a domain name to another domain name. DNS uses the second name to look up the address. Users still see the first domain name in their browser. For example, you could map contoso.com to your webapp.azurewebsites.net.

Nslookup

nslookup is to lookup DNS entries.