nc -vz servername portnumber
ubuntu:~$ nc -vz 172.16.88.109 23
Connection to 172.16.88.109 23 port [tcp/telnet] succeeded!
Another option is telnet:
telnet servername portnumber
ubuntu:~$ telnet 172.16.88.109 23
Trying 172.16.88.109…
Connected to 172.16.88.109.
Escape character is ‘^]’.