macOS 10.13: Telnet is gone.. Use NC
With the demise of telnet in macOS 10.13 High Sierra it is recommended we use the nc command.
Here is an example os the usage:
nc -z 192.168.110.220 8443
The -z flag can be used to tell nc to report open ports, rather than initiate a connection
You can also test a range:
nc -z 192.168.110.220 1-8443
View the man page for nc: