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Serve static content from a cookieless domain
Overview
Serving static resources from a cookieless domain reduces the total size of requests made for a page.
Details
Static content, such as images, JS and CSS files, don't need to be accompanied by cookies, as there is no user interaction with these resources. You can decrease request latency by serving static resources from a domain that doesn't serve cookies. This technique is especially useful for pages referencing large volumes of rarely cached static content, such as frequently changing image thumbnails, or infrequently accessed image archives. We recommend this technique for any page that serves more than 5 static resources. (For pages that serve fewer resources than this, it's not worth the cost of setting up an extra domain.)
To reserve a cookieless domain for serving static content, register a new domain name and configure your DNS database with a CNAME record that points the new domain to your existing domain A record. Configure your web server to serve static resources from the new domain, and do not allow any cookies to be set anywhere on this domain. In your web pages, reference the domain name in the URLs for the static resources.
If you host your static files using a CDN, your CDN may support serving these resources from another domain. Contact your CDN to find out.
Recommendations
Enable proxy caching.
For resources that rarely change, set caching headers for browsers and proxies. Because cookies will not be sent for these resources, there is no risk that proxy caches will cache user-specific content.
Don't serve early loaded external JS files from the cookieless domain.
For JavaScript referenced in the head of the document and needed for page startup, it should be served from the same hostname as the main document. Because most browsers block other downloads and rendering until all JavaScript files have been downloaded, parsed and executed, it's better to avoid the risk of an additional DNS lookup at this point of processing.
Example
Many Google properties, including News and Code (this site), serve static resources, such as JS files and images, from a separate domain,
www.gstatic.com 
. No cookies can be set on this domain. For the News homepage at news.google.com, you can see the cookie in the request header in this screen shot:
But for
www.static.com , which is used to serve the News logo .gif file, no cookie headers appear in the request or response:
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