The problem created by network address translation – Sierra Wireless MP 875 User Manual

Page 55

Rev 1.1 Aug.07

Network

Internet

Firewall

Sending Data to Your Network Application

The problem created by Network Address
Translation

Note: A similar problem may

Many service providers use NAT (Network Address Trans-

occur if NAT is in use on your

lation) which effectively creates a firewall in front of the

network. The host application

network. Inside the network, the service providers assign

may reside on a server that has

private IP addresses to registered devices, since public IP

a private IP address. You must

addresses are not needed to route data within the network.

configure the MP modem to

(This saves the service provider the expense of having a large

send data to a server on your

pool of public IP addresses.) A public IP address is substituted

network with a public IP address,

for the private IP address when data packets pass through the

and configure the server to

gateway between the network and the Internet. This public IP

recognize the MTP data and

address might be used on data transmissions from many

route it appropriately.

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 5-1: 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.