Is xxx.domain.com and www.domain.com the same?

[cd34]

Another possible reason to use wildcards would be to avoid IE's 2 socket connection limit when hitting web sites. Briefly, IE will put 2 image requests to a hostname, and queue up the rest to be fetched as the 4 requests complete. If you use multiple hostnames, i.e. img1.domain.com, img2.domain.com, img3.domain.com, IE will request 4 connections per hostname -- even though the requests are being handled by the same server.

http://www.die.net/musings/page_load_time/