Chat question : Which architecture? IRC/UDP???

Hi all,

I am new to chat applications.

I want to build a chat system who's design can handle many
100,000's of users.

Server load, and portability (ie firewalls) is a major issue, so
I want to keep comm's traffic to an absolute minimum.

I realise this may seem like a dumb question to you, but I'm a
relative newbie to the chat game! :)

P.S. I also noticed the IRC port is 6667, can some sort of
tunnelling approach work with IRC to get around the firewall
issues?

Thanks again,

Jason.