Apple, in their ultimate wisdom, have removed telnet and ftp client on macOS High Sierra.
It is true. You should not use these commands anymore to do unencrypted management on switches.
For a network administrator these commands are extremely important to do debugging and troubleshooting.
Get your telnet back:
First, install Xcode from the App Store.
Then open a terminal and get the ⇒GNU inetutils using curl :
curl http://ftp.gnu.org/gnu/inetutils/inetutils-1.9.4.tar.gz -o inetutils-1.9.4.tar.gz
Then compile it yourself:
tar xvzf inetutils-1.9.4.tar.gz cd inetutils-1.9.4 ./configure make
The next thing would be
sudo make install
then all clients are in /usr/local/bin.
Preferably you install only telnet and ftp. make install installs a whole bunch of commands, which do not work as expected afterwards.
To get only telnet and ftp, simply copy the executables.
cd inetutils-1.9.4/telnet sudo cp telnet /usr/local/bin
cd inetutils-1.9.4/ftp sudo cp ftp /usr/local/bin
If you require telnet for troubleshooting, ⇒netcat is a perfect alternative.