The netstat command displays a list of hosts connected to your current server. You can only telnet or rlogin to servers in the netstat list. (You can also dial into hosts if you know their phone number.)
The telehack host only has only eleven hosts listed for each user. That is because hosts in telehack are arranged in a web. Each host you can connect to has it's own netstat full of other hosts you can connect to. In this way, you can connect to any of telehack's 26,631 hosts.
netstat also displays information about the remote host. It displays an * next to any host you have login on (via running porthack) or a ! if you have root on the host, hostname, organization, OS, and location.
So let's say you want to get to uunet, and it's not in your netstat. How do you get to it?