CalAmp Viper 100 User Manual page 55

4.4.2.2 NAT OVERVIEW
NAT is the process of modifying network address information in datagram (IP) packet headers while in transit across a
traffic routing device for the purpose of remapping one IP address space into another. Most often, NAT is used in
conjunction with network masquerading (or IP masquerading) which is a technique that hides an entire IP address space,
usually consisting of private network IP addresses, behind a single IP address in another, often public address space. This
mechanism is implemented in a routing device that uses stateful translation tables to map the "hidden" addresses into a
single IP address and then readdresses the outgoing Internet Protocol (IP) packets on exit so that they appear to originate
from the router. In the reverse communications path, responses are mapped back to the originating IP address using the
rules ("state") stored in the translation tables.
As described, the method enables communication through the router only when the conversation originates in the
masqueraded network, since this establishes the translation tables. For example, a web browser in the masqueraded
network can browse a website outside, but a web browser outside could not browse a web site in the masqueraded
network. Most NAT devices today allow the network administrator to configure translation table entries for permanent use.
This feature is often referred to as "static NAT" or port forwarding and allows traffic originating in the 'outside' network to
reach designated hosts in the masqueraded network.
Figure 30 – Basic NAT Operation
In our example, Host 1 sends a packet to Host 2. The Host 2 device does not see the private IP address of Host 1. When Host
2 sends a reply to Host 1, Host 2 uses the destination IP address 172.31.5.1, which is translated back to the appropriate
destination IP address by the NAT enabled device (see Figure 30).
NAT does a lot more than just translation of the source IP address. For the UDP and TCP protocol, NAT will also translate the
source port numbers. Special handling is also done for more specific protocols like FTP (port 21) and Modbus (port 502).
4.4.2.2.1 NAT ON VIPER
In a Viper SC, it is normally used on the WAN side of an IP network to hide local IP addresses from an external IP network.
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