What you do is determine from the "DHCP Server" on your [home] network, what ip address it gave your Tivo.

e.g. if your DSL router also provides DHCP, then it should have somewhere in its configuration menus etc where it can show you what DHCP addresses are in use allocated by it and by what "hostnames" they are e.g. Tivo may show up as "tivo" in the DHCP Server list with a particular IP address.

If you don't have a DHCP server in your home network, then you'll need to set one up/(enable it on your router) or the Tivo can't get an IP address and thus can't use the network.

Then once you know the ip address you can use that to telnet in to your Tivo.

Once you know the hostname for the tivo that the Tivo registers in DHCP when it gets an IP address. You should be able to use that name in all future telnet sessions instead of the IP address provided you have DNS also running on your DHCP server [or your DHCP and DNS servers know about each other].