Where it says hostname here, do browsers consider subdomains to be seperate hostnames, or the same hostname?

They should consider them to be seperate hostnames. The problem related to cookies though is if you have “.mydomain.com” as the cookie domain, the browser will send the cookie on each request to mydomain.com and all subdomains.

Aside from how intermediate proxies respond to that, it can be a significant chunk of bandwidth you’re losing to those requests - if the cookie header is, say ~ 500 bytes (about the size of Sitepoint’s cookies), for a page that requires say 10 images, 1 external Javascript script and 1 CSS file, all on the same domain, that’s 6 kbytes of bandwidth spent, just on having cookies sent for static content requests - adds up when you have a lot of visitors.

Does the same theory apply for dynamic content as well? I know the cookie issue might be a hurdle if people are using them as session trackers.

Not sure I understand the question.

Images loading from another domain has more to do with letting one server/domain be dedicated to serving static content, while the main server does server side processing.

That’s a good point. Servers like TinyHTTPD often get employed to do this.

A single Apache child + mod_php, for example, represents a significant block of memory - the principle is don’t waste those child processes on serving “dumb” content. And, specific to PHP, it can also be important when you’re using persistent resources that stay in memory - George Schlossnagle makes that point well here;

If the average page in your Web application contains nine images, then only ten percent of the requests to your Web server actually used the persistent connections they have assigned to them. In other words, ninety percent of the requests are wasting a valuable (and expensive, from a scalability standpoint) Oracle connection handle. Your goal should be to ensure that only requests that require Oracle connectivity (or at least require dynamic content) are served off of your dynamic Web server. This will increase the amount of Oracle-related work done by each process, which in turn reduces the number of children required to generate dynamic content.

The easiest way to promote this is by offloading all of your images onto a separate Web server (or set of Web servers).

I must admit though, I did wonder why sites like eBay load images etc from another domain. Figured it was some some kind of consolidation excercise with PayPal.

In the end, no matter how many connections the client opens to your infrastructure, they are still limited by their ISP connection. But I digress, that’s client performance vs server performance.