Why does nslookup return two or more ip address for yahoo.com or microsoft.com?

I am newbie with dns

What is the exact reason that some sites report two or more ip address while others report single ip?

1) Is it feature or error in configuration?
2) Is it something related to backup address or fail-over config?
3) Can we setup this on LAN for learning?

Any help, clarification or guidance is welcome.

Solution 1:

Multiple A records for a single host lookup is commonly known as round-robin DNS.

1. It is a feature
2. It is related to high-availability and load-sharing
3. Yes

Be advised early on that round-robin DNS is at best a mixed-bag. For more information, please consult the following resources:

http://www.zytrax.com/books/dns/ch9/rr.html

http://www.zytrax.com/books/dns/ch7/queries.html#rrset-order

http://www.rightscale.com/blog/enterprise-cloud-strategies/dns-load-balancing-and-using-multiple-load-balancers-cloud