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Port check tool is a free utility tool for remotely access verifying if a port is open or closed. It is useful to users who wish to verify port forwarding if it`s block by the ISP/router.

Port Forwarding.
Port forwarding or port mapping is an application of network address
translation (NAT)that redirects a communication request from one address
and port number combination to another while the packets are traversing a network gateway,
such as a router or firewall. This technique is most commonly used to make services
on a host residing on a protected or masqueraded (internal) network available
to hosts on the opposite side of the gateway (external network),
by remapping the destination IP address and port number of the communication
to an internal host.

Purpose
Port forwarding allows remote computers (for example, computers on the Internet)
to connect to a specific computer or service within a private local-area network (LAN).
In a typical residential network, nodes obtain Internet access through a DSL or cable modem
connected to a router or network address translator (NAT/NAPT). Hosts on the private network
are connected to an Ethernet switch or communicate via a wireless LAN.
The NAT device’s external interface is configured with a public IP address.
The computers behind the router, on the other hand,
are invisible to hosts on the Internet as they each communicate only with a private IP address.
When configuring port forwarding,
the network administrator sets aside one port number on the gateway for the exclusive
use of communicating with a service in the private network, located on a specific host.
External hosts must know this port number and the address of the gateway
to communicate with the network-internal service. Often,
the port numbers of well-known Internet services, such as port number 80 for web services (HTTP),
are used in port forwarding, so that common Internet services may be implemented on
hosts within private networks.