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 .
Step 3: Make sure that you don't make hacking instructables at school
Step 4: Start the telnet program
Step 5: Start the connection
You will want to use the IP address that you found port 23 open on, or the game, nethack.alt.org.
For example, using nethack.alt.org as our telnet server you would type "o nethack.alt.org 23".
If you don't type a port number, it will assume you mean 23.
Now hit enter to connect.
Step 6: Play with the device you connected to
One of the most easy thing to hack via telnet, is a Cisco router. All Cisco routers have a telnet server built in. To learn the Cisco "language" go to this website.
When connected to a Cisco router you will get something similar to the picture below. Type "enable" and you can use the privileged mode. After entering privileged mode, type "config t" to configure the router.
Step 7: Jumping
Let me remind you that while it is not illegal to use telnet on stuff you own or have permission to, it is very illegal to use telnet to harm systems you do not have permission to. This guide was meant as a tool to hack into your own stuff. Many Linksys routers have telnet in them. People use the telnet as a means of making the power sent to the antennas (wireless routers) stronger. This in turn, makes the signal stronger.
PS. The WRT54G is the easily hackable Linksys router.