Viper SC+™ IP Router for Licensed Spectrum PN 001-5008-000 Rev. C
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APPENDIX G
– NAT OVERVIEW
NAT (Network Address Translation) 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
masquerading 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 81 – Basic NAT Operation
In the above 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, as shown in the preceding figure.
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).
