If you don't have telnet or nc installed and want to quickly test firewall traffic to a server and specific port you can try this. It needs a new enough bash but still pretty quick and handy.

Good test port is open

<pre class="brush: bash; title: ; notranslate" title="">
$ timeout 1 bash -c 'cat < /dev/null > /dev/tcp/172.18.10.66/1521'
$ echo $?
0

Port not open

<pre class="brush: bash; title: ; notranslate" title="">
$ timeout 1 bash -c 'cat < /dev/null > /dev/tcp/172.18.10.66/15'
$ echo $?
124

Good test port is open to google FQDN

<pre class="brush: bash; title: ; notranslate" title="">
$ timeout 1 bash -c 'cat < /dev/null > /dev/tcp/google.com/80'
[opc@ocilxeasdbt02 ~]$ echo $?
0