If you are trying to connect to an i5-iSeries-AS/400, you probably want a telnet client that handles 5250. This is called a TN5250 client.
Mochasoft (www.mochasoft.dk) offers an excellent shareware TN5250 client that is only $25 per client.
But Sourceforge has a true open-source, freeware TN5250. Significant authors are Carey Evans, Jason M. Felice, Scott Klement, and Michael Madore. You can download it and learn more at tn5250.sourceforge.net
Since it's a little confusing to download it there, you may download a WIndows version here
It is small (less than 1 mb) and installs very quickly.... and it is free.
I suggest two configuration changes to make:
After installation, you should have a desktop icon named TN5250.
Right-click on that icon and then click on properties. Add the characters
(that is an underscore chracter between "+resize" and "fonts") This will allow you to resize the window.
In the folder "Program Files", in the sub-folder "TN5250" is a file named "TN5250rc". Change it (one way is to right-click, selct open-with and select notepad or Wordpad)
Make it look something like this:
(Instead of 192.168.0.1, enter your host ID or TCP/IP address)
The font_80 line changes the font to MS Gothic and the 12x24 defines the width and height of the window. Change those until the window is the size that you like.
That's it. Double-click the TN5250 icon and you should see an i5-iSeries-AS/400 display terminal session.
This application is very full-featured. It also includes a Linux version.
Go to TN5250.sourceforge.net to learn more.