Device Located Behind Nat; Configuring Nat Translation Per Ip Interface - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 13 Network
The design of SIP creates a problem for VoIP traffic to pass through NAT. SIP uses IP addresses
and port numbers in its message body. However, the NAT server is unable to modify the SIP
messages and thus, can't change local addresses to global addresses.
This section discusses the device's solutions for overcoming NAT traversal issues.

Device Located behind NAT

Two different streams of traffic traverse through NAT - signaling and media. A device located
behind NAT that initiates a signaling path has problems receiving incoming signaling responses, as
they are blocked by the NAT server. Therefore, the initiating device must inform the receiving
device where to send the media. To resolve this NAT problem, the device provides the following
solution:
■ NAT Translation table, which configures NAT per IP network interface - see
Translation per IP
If NAT is not configured, the device sends the packet according to its IP address configured in the
IP Interfaces table.
The figure below illustrates the NAT problem faced by SIP networks when the device is located
behind a NAT:

Configuring NAT Translation per IP Interface

The NAT Translation table lets you configure up to 32 network address translation (NAT) rules for
translating source IP addresses into NAT IP addresses (global -public) when the device is located
behind NAT. The device's NAT traversal mechanism replaces the source IP address of SIP
messages sent from a specific VoIP interface (Control and/or Media) in the IP Interfaces table to a
public IP address. This allows, for example, the separation of VoIP traffic between different ITSPs
and topology hiding of internal IP addresses from the "public" network. Each IP network interface
(configured in the IP Interfaces table) can be associated with a NAT rule, translating the source IP
address and port of the outgoing packet into the NAT address (IP address and port range).
The following procedure describes how to configure NAT translation rules through the Web
interface. You can also configure it through ini file [NATTranslation] or CLI (configure network
> nat-translation).
Interface.
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Mediant 4000 SBC | User's Manual
Configuring NAT