To test connectivity with ports accepting TCP connections, I
normally use:
$ telnet <machine> <port>
It has the additional feature that once connected, you can interact with the service using the protocol directly. For instance when connecting to a web server you can perform HTTP GETs.
However, with services listening for UDP only, like
NTP, telnet
doesn't work. You can then use
netcat:
$ nc -v -u -z -w 3 ntp.example.com 123
Connection to ntp.example.com 123 port [udp/ntp] succeeded!
Once you get used to using nc, you may even start using it for
testing TCP connectivity too:
$ nc -v -z -w 3 example.com 80
Connection to example.com 80 port [tcp/http] succeeded!
Easy when you know how :-)
