DNS is also used to perform load distribution among replicated servers, such as replicated Web servers. Busy sites, such as cnn.com, are
replicated over multiple servers, with each server running on a different end system and each having a different IP address. For replicated Web servers, a set of
IP addresses is thus associated with one canonical hostname. The DNS database
contains this set of IP addresses. When clients make a DNS query for a name
mapped to a set of addresses, the server responds with the entire set of IP
addresses, but rotates the ordering of the addresses within each reply. Because a
client typically sends its HTTP request message to the IP address that is listed
first in the set, DNS rotation distributes the traffic among the replicated servers.