It depends on the network. Many networks are set up where you can use something like us.undernet.org instead of irc.undernet.org to connect to a US/Canada server or eu.undernet.org to connect to a European server. Or, you can specify the server you want... something like: some.server.us or some.server.se . The server names are usually available on the network's website, or if they are servers in the default server list in mIRC, you can find out the information from there.
