Connect it to the 'net. If you don't want other people besides your own servers to be able to access it and use it as a cacheing server, then firewall off port 53, in and out, tcp/ip and udp, from everything except our own servers.
hmm i have build my own internal nameservers but now i must have hit the 100.000 queries sometimes.. and now these nameservers are i guess "blocked" again by dnsbl.org ;(
so i need the license, or remove dnswl.org or build yet another set of nameservers on another ip and use that only for a few servers ;(
as i see allowing people to do mailinglists could trigger this.. (especially if a few combined will trigger the 100.000 limit set by dnswl.org ;(