Setting up a private Network without interfering with my ISP

Hi all,

I was wanting to set up a test network at home, with DNS, Kerberos,
Mail, Firewall, etc and use an internal domain name (not internet
accessable), but still be connected to the Internet through my ISP
(Comcast cable).

My main concerns are: 1) not letting my internal network name out into
cyberspace ( I try to have good manners in that regard) and 2) having
my internal network use my DNS server, but internet browsing still go
to Comcasts DNS servers.

I don't really need assistance in setting it up, just some pointers or
advice on any "gotchas" so that I don't mess up things with my ISP and
so that I can still browse.

Thanks

matt