Network engineers often use telnet to establish a connection to a service on a remote server. Apple dropped the telnet and ftp client in macOS High Sierra. First I was very disappointed, but realistically telnet is history.
Instead of trying solutions to get telnet back, look forward and start using netcat.
Connect to a webserver and get the headers:
nc -v www.microsoft.com 80
Then type your GET request for / with a minimal host header.
GET / HTTP/1.1 [enter] Host: example.com [enter]
and you get the result
Connection to www.microsoft.com port 80 [tcp/http] succeeded! GET / HTTP/1.1 Host: www.example.com HTTP/1.1 503 Service Unavailable Server: AkamaiGHost Mime-Version: 1.0 Content-Type: text/html Content-Length: 268 Expires: Sun, 18 Feb 2018 15:09:36 GMT Date: Sun, 18 Feb 2018 15:09:36 GMT Connection: keep-alive <HTML><HEAD> <TITLE>Service Unavailable</TITLE> </HEAD><BODY> <H1>Service Unavailable - DNS failure</H1> The server is temporarily unable to service your request. Please try again later.<P> Reference #11.7efa6d68.1518966576.17cd556 </BODY></HTML>
This method can be used for all ports. You can also get the headers for mail servers, ssh servers.
If you only want to know if the port is open, simply use:
nc -vz www.microsoft.com 80
You can also use netcat to verify if UDP ports are open:
nc -vz -u 188.8.131.52 53
And netcat can be used as a port scanner:
nc -vz <hostname or ip address> 1-1000