When you register the domain name, you must have dns servers set up to handle queries. If you do a whois on
Network Solutions (now VeriSign) for worldcom.com and mci.com, you will see both have the same dns servers.
You can see differences in the two when you go into nslookup.
Set type to mx and put in their domain names. They share the same MX records but the DNS servers have changed (two extra for wcom.com).
To learn more, check out "DNS and Bind" by By Paul Albitz, Cricket Liu (
Oreilly, visit
Mr. DNS, or do a search on goggle.com. The is an abundant supply of knowledge about the subject on the Internet.
One answer to your question can be found after searching on
Mr. DNS