This is done by checking the format of the IPv4 address. This looks familiar This is interpreting a single number as an IPv4 address. This means 'treat this number as octal' which is base After 07 is the number written as 8 in decimal base 10but there aren't any digits left. I found this video Mainframe operating system MVS 3. However I am glad it does work the other way. The TK4 version didn't seem to work on os x and is probably preconfigured for Linux and not os x. Then I could fill in username and password. And I installed with sudo apt install c the telnet terminal emulator in the linux vagrantbox. On mac os x I have installed Vagrant ,and a vagrantbox for linux I downloaded from The MVS 3. With macports I have installed Hercules and c Edit 1. Retrocomputing Stack Exchange is a question and answer site for vintage-computer hobbyists interested in restoring, preserving, and using the classic computer and gaming systems of yesteryear. By using our site, you acknowledge that you have read and understand our Cookie PolicyPrivacy Policyand our Terms of Service.