Error in communication between Reader and PC via TCP/IP protocol

Could you please take a look on my explanation to help me?

Our system has 2 things:

1. RFID Reader (used to read ID tag)
2. PC

Two devices are connected via TCP/IP protocol with following setup:

Reader: IP address: 172.21.16.3;

Port 10001

Subnet Mask: 255.255.255.0

Default gate: 127.21.16.254

PC: IP address: 172.21.16.4

Subnet Mask: 255.255.255.0

Default gate: 127.21.16.254

The first connection: RFID reader and PC are connected directly. On PC, we used Hyper Terminal to collect data from RFID reader with the setup in attached file.

The result: PC and RFID can communicate, PC received data from RFID reader.

We used scan port soft to check IP and port. Both of them were informed to be alive.

The second connection: RFID reader and PC are connected via network (the system has 1000PC). On PC, we used Hyper Terminal to collect data from RFID reader with the above setup:

The result: PC and RFID can not communicate with warming “unable connect”

We used scan port soft to check IP and port. Port 10001 was informed to be closed.

Questions:

1. May I know why port 10001 is closed?
2. What is the factor make port 10001 closed?
3. How can I show this problem?
4. What is the traffic internet?
5. How can we know the network is traffic?
6. How can we show the traffic internet problem?