I’m trying to figure out whether it would be worthwhile to spread image requests across multiple sub-domains. [This article](link broken) for example says:
Most browsers can only make two requests at a time, so the browser will request two files, download them and then move on to the next two. The more HTTP requests, or separate components a page requires to display properly, the longer the user will have to wait.
When they say most, which browsers in particular? Is that number related to the number of concurrent XMLHttpRequests, per this question?
There are a lot of things to consider here. In most situations, I would only choose one cookieless domain/subdomain to host your images such as static.mywebsite.com. And ideally static files should be hosted by a CDN, but that’s another story.
First of all, IE7 allowed only two concurrent connections per host. But most browsers today allow more than that. IE8 allows 6 concurrent connections, Chrome allows 6, and Firefox allows 8.
So if your web page only has 6 images, for example, then it’d really be pointless to spread your images across multiple subdomains.
So let’s say you have 24 images on a page. Well, few things in life are free and there’s such a thing as death by parallelization. If you host your images in 4 different subdomains, then that means that every single image could theoretically be downloaded in parallel. However, it also means that there are 3 additional DNS lookups involved. And a DNS lookup could be 100 ms, 150 ms, or sometimes longer. This added delay could easily offset any benefit of parallel downloads. You can see real-world examples of this by testing sites with http://www.webpagetest.org/
Of course the best solution is to use CSS sprites when possible to cut down on the number of requests. I talk about that and the inherent overhead of every request in this article and this one.
UPDATE
There’s an interesting article from Steve Souders on the subject of sharding domains…
http://www.stevesouders.com/blog/2009/05/12/sharding-dominant-domains/
Note however that this was written in 2009. And in 2011 he posted a comment…
You should also keep in mind that the big reason it’s even necessary for the big sites like Yahoo and Amazon to do domain sharding is that their sites are so dynamic. The images are attached to products or stories which are displayed dynamically. So it’s not feasible for them to use CSS sprites as aggressively as would be optimal.
A site like StackOverflow, however, is light on these sorts of images and they have cut down on the number of requests so much that they don’t need to do sharding. A big step towards making that happen is their usage of this sprites.png image…
http://cdn.sstatic.net/Sites/stackoverflow/img/sprites.png?v=5
UPDATE #2
Steve Souders posted another update on domain sharding. He repeats much of what I’ve already mentioned. But the thing that stood out was SPDY and how that should affect your decision.
UPDATE #3 (February 2018)
As Dean mentioned in the comments below, CSS sprites aren’t really buying you very much now with HTTP/2 being supported in modern browsers. But you do have to get an SSL certificate, set up your site to work with HTTPS, and ensure your web server is configured for HTTP/2. Either that, or use a CDN that already has all of that set up for you. Once you’ve done all of that then you can probably skip both CSS sprites and domain sharding.