In large scale environments several web servers are often represented by a single public IP address. In these cases the requests are load balanced among the group of servers. This can be achieved through DNS entries or more often managed by a hardware device such as F5 Networks Big-IP. 
The information stored on a DNS server is broken into DNS Zones. Zone Files, typically one file for each domain the server is responsible for (see Authority section page ), come in two formats. The Primary Master server for the domain (see Name Servers section page ) has the original file, often directly edited by the DNS Administrator. This file can include comment lines and spacing for readability. There is also an auto generated version of the zone file created on the master and sent to the slave servers. There are a few mechanisms related to how the slaves are notified of the change but what is important is that changes are automatically distributed to all the servers answering DNS requests for the given zone.