How to check TCP ports reachability ?

As we know already - permit/deny our interested traffic based on port numbers using Extended ACL, I get into the topic straightway.

Remote host reachability test can be done using ICMP echo reply.

But, How can we do the port based reachability test?

We can use extended telnet command to check port based reachability of remote host as well as localhost.

I believe, you might have confused, why we need to check local host?

All network applications are working on TCP/UDP ports. It means, the server-client communications, server-server communications are basically happening on TCP/UDP ports.

So, basically the required ports of remote-host should be reachable for the local-host. Hence, in secured communication we are having lot of hardware/software firewalls in the network, we need to open the specific ports need for the application.

The remote-host port reachability can be verified by the following extended telnet command.

telnet <remote-host interface IP> <port-number>

e.g., telnet 192.168.1.2 443

To sort out remote host - local Antivirus Software port blocking can detected by the following method.

Some applications need two-way communication. So, the port should be reachable from remote to local as well.

The local-host port reachability can be verified by the following extended telnet command.

telnet <local-host interface IP> <port number>

e.g., telnet 192.168.1.1 21