Hm... Well, I've tried with setting mIRC to use only port 1024 and I don't have any form of router, yet it will send multiple things at the same time.
Your description of the NAPT sounds just like what I was told about routers having issues resuming, so that's definitely what I was thinking of.
Ok, I looked up the limit information. It's regarding the number of connections per second. I thought it was overall. It's for SP2 and is for both Home and Pro versions. Anyhow, here's a link I found about it:
http://www.speedguide.net/read_articles.php?id=1497
