This was titled like this to be entered into the Burning Questions Group. I understand it to mean, "How to hack via Telnet," so that's what this instructable is about.
Step 1: Download Stuff
Step 2: Using Zenmap (nmap's GUI)
If you don't know the ip address, go to the computer you want to access (I will assume you have the legal rights to whatever you are trying to access), open a command line and type ipconfig. You can try to hack devices too sometimes. Routers and switches sometimes have telnet. Try scanning these too.
Say you want to scan a range of IP addresses, then you would type the beginning IP address followed by (no spaces) a dash ( - ) and the end of the last IP address. For example, if you wanted to scan 192.168.1.100 to 192.168.1.299 then you would type 192.168.1.100-299 .
In the "Profile" menu of Zenmap, select "Intense scan" if it is not already selected. Click scan and watch as the program scans the computer or device for open ports. Watch the nmap output until you see some words in green. These are the open ports on the computer or device. If port 23 is open, your device is hackable.
Many servers and routers have port 23 open.
If you can't find a device with Telnet enabled, use nethack.alt.org .