Using telnet command :
Syntax : telnet <Host_name> <port_no>
telnet test.openspecimen.org 80
Use the above command from outside the server machine. If port 80 is open. "Connected to test.openspecimen.org." message will be displayed on the terminal.
Way 2 :
Listing down iptables rules. Below is the command to list iptables rules :
Check if there is any rule set for port 80, in the same row if under 'Target' column DROP option is present then port 80 is blocked.
Way 3 :
The below commands will display open ports of the server.
|Ubuntu||sudo ufw status|
|CentOS / RHEL||sudo firewall-cmd --list-all|