If you got the console terminal that was with the machine just log on and change the IP address(es) to ones in the range of your home network – CFGTCP ->opt 1.
If you don’t have the console, you can set the subnet mask on your desktop or laptop to 0.0.0.0 then telnet to the AS400 ip address. Then you can change the address as above. You’ll probably lose connection when you do that, but you should then be able to re-issue the telnet command with the new As400 address.
Remember to change the subnet mask back to its original value on the desktop or laptop.