Many user doubt: Why GPRS / GSM modems only can go into a public IP computer? Can it work at a campus network/corporate network?
Answer: Yes, it can. But it need cooperate with campus network/corporate network. Detail explanation as below:

GPRS / GSM modems connect to network is send a TCP/UDP connection, during connection and communication progress, we make a phone call as a example to explain it: (Make phone call is similar with GPRS / GSM modems connection)

Detail picture as below: Two company A and B, general phone number are 123 and 456, two company extension number are 01, 02, 03 and 05, 06, 03. So now if company A extension number 01 want to call company B extension number 03, how to do it?

Obviously, he should call company B general phone number 456 when it called then transfer extension number 03, or when dialing directly dial 456-03 is OK. If he just dial extension number 03, the telephone only can come into company A extension number 03 and it can’t come into company B.

Base on above example, on the internet, detail as below:

Public IP same as general phone number, it can dial number directly. LAN IP like an extension number, it is public IP port number, in other words Public IP mapped to a local area network (LAN) computer port, it is LAN PC extension number.Port forwarding same like assign extension number to LAN PC. DHCP like someone often change telephone number. DDNS like on the network, it have a device to record change number.

When GPRS / GSM modems connect network, it is under mobile LAN network, IP address is LAN IP, like telephone network in the extension number and that extension number is not fixed, it is constantly changing. Every time when connect, it will obtain a new extension number, if user want to GPRS / GSM modems connect to PC directly, so PC must have a public IP (like fixed general number), or at lest have a public IP extension number(port forwarding), otherwise, it can not communication.

So how to obtain a public IP? Normal network user, use broadband or ADSL to surf the internet. This way, we obtain a dynamic public IP. So now it need set dynamic domain name, put this dynamic domain name to GPRS / GSM modems, GPRS / GSM modems could seek corresponding IP address.

If network user IP is not public IP, user PC need connect one server whom have public IP, tell server detail number, GPRS / GSM modems also send data to server, so server reach data transfer function.