What is "domain sharding"

It's a term that Steve Souders uses to describe intentionally splitting web page resources across multiple domains to make pages load faster.

Steve Souders has done extensive research on improving front-end performance of website. He has shared his findings on "domain sharding":

• Most of the U.S. top ten web sites do domain sharding.
• The optimal number of domains to shard across is 2-4. After 4 domains, response time degrades
• Domain sharding is especially useful for older browsers as they are restricted to just two parallel downloads per hostname. The newer browsers however do better. Opera 9+ and Safari 3+ do four downloads per hostname. Internet Explorer 8, Firefox 3, and Chrome 1+ do six downloads per hostname.

Also see:
- stevesouders.com: Domain Sharding revisited (September 2013)
- HOW TO improve the front-end performance of a website
- HOW TO speed up a ASP.NET AJAX web app