What's A Network Address Translator - Polycom ViaVideo User Manual

Appendix

What's a Network Address Translator?

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A NAT is a set of services on the router that allow you to operate a
local network of devices, each with its own locally unique IP address,
while communicating to the Internet through a router which presents
a single, globally unique IP address to the outside world. Most small
business and home office (SOHO) routers provide NAT services.
The NAT ensures that outgoing packets have a globally valid IP
address, and that incoming packets go to the right device by
translating between the internal IP address and the external IP
address, owned by the router. This also adds an extra level of
security by effectively masking the internal network behind a single
external IP address.
A simplified explanation of how this translation takes place is that
when an internal device sends an outgoing packet to the router, the
NAT service either notes the internal IP address or the port number
of the device, and attaches an identifier to the external IP address of
the outgoing packet, substituting the router's globally unique IP
address for the device's internal IP address. When the answering
packet comes back, the NAT notes the indentifier, re-assigns the
packet to the device's internal IP address, and sends it on its merry
way.
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Network Connection
ViaVideo User's Guide