Sierra Wireless MP 875 - Manual (Page 69)

Sending Data to Your Network Application
Rev 1.5 Jul.08 69
The problem created by Network Address
Translation
Note: A similar problem may
occur if NAT is in use on your
network. The host application
may reside on a server that has
a private IP address. You must
configure the MP modem to
send data to a server on your
network with a public IP address,
and configure the server to
recognize the MTP data and
route it appropriately. Many service providers use NAT (Network Address Trans‐
lation) which effectively creates a firewall in front of the
network. Inside the network, the service providers assign
private IP addresses to registered devices, since public IP
addresses are not needed to route data within the network.
(This saves the service provider the expense of having a large
pool of public IP addresses.) A public IP address is substituted
for the private IP address when data packets pass through the
gateway between the network and the Internet. This public IP
address might be used on data transmissions from many
different devices. Therefore, the network server is only aware
of the substituted public IP address. If the server replies to this
address, the network cannot identify the device for which the
data was intended.
Assume, for example, an MP modem is registered on a
network. The MP modem is assigned a private IP address of
10.95.236.1. When the MP modem sends MTP data to the
network server, a public IP address of 24.7.6.30 is substituted at
the gateway to the Internet. When the network server receives
the data, the host application is only aware of the public IP
address, 24.7.6.30. However, if the network server sends data
back to the IP address 24.7.6.30, the network has no way of
identifying the MP modem for which it is intended and is
unable to route the data appropriately.
Figure 7-1: An MP modem connected to a network server. In this scenario, the
IP address is “NAT’d” from 10.95.236.1 to 24.7.6.30. The network server is
only aware of the address 24.7.6.30, but if it sends data to that address the
network cannot route the data to the MP modem.
Network Internet
Firewall