The WWW prefix (short for World Wide Web) that precedes URL addresses is an indication that the Web address exists on the vast network of the World Wide Web. Sometimes in a URL the "WWW" is followed by a number, such as "WWW1" or "WWW2."
The number that follows the "WWW" indicates that the data being retrieved by the Web browser is gathering the information from a different Web server than the one that serves the typical "WWW" address.
Web sites, especially dynamic Web sites, that handle large amounts of traffic often need more than one server to accommodate the many requests they receive as one server often cannot handle the multitude of requests. The numbers that follow the "WWW refer to different Web servers, often as elements of a server farm, that all contain the exact same information. The servers are used in coordination with each other for load balancing.
An example of this system is www.google.com, which uses multiple servers to handle all its traffic. Sometimes the user's physical location determines which server receives the routed requests, and sometimes the different servers are used when one or more of the servers need to be taken offline in order to be updated with current information.
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