Lots of things changed under the hood in MacOS high sierra. One of those was to enable a sandbox like environment and to remove insecure communication protocols. This breaks things like console communication to the network modeling and virtualization platform Eve-NG. It’s fairly trivial to re-enable it, however. This can be accomplished by doing the following steps.

Install Homebrew. Open your favorite terminal application (I like to use iTerm2 (also installable via homebrew), but Terminal works fine.)

1. Reboot your Mac and hold the CMD + R keys 
2. When presented with the recovery options, click Utilities at the top and choose Terminal 
3. Type; csrutil disable 
4. Reboot as normal 
6. terminal and type; 
brew install telnet 
sudo ln -s /usr/local/bin/telnet /usr/bin/telnet 
7. Again, Reboot your Mac and hold the CMD + R keys 
8. When presented with the recovery options, click Utilities at the top and choose Terminal 
9. Type; csrutil enable 
10. Reboot as normal 

You’re done. You can have telnet for your internal communication to Eve-NG consoles. Don’t use it to talk to production network gear, because it’s not 1998.