Network Address Translation Support; Device Located Behind Nat - AudioCodes Mediant 3000 User Manual

Media gateway & enterprise session border controller (e-sbc)

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User's Manual

Network Address Translation Support

Network Address Translation (NAT) is a mechanism that maps internal IP addresses (and

ports) used within a private network to global IP addresses and vice versa, providing

transparent routing to end hosts. The primary advantages of NAT include (1) reduction in

the number of global IP addresses required in a private network (global IP addresses are

only used to connect to the Internet) and (2) better network security by hiding the internal

architecture.

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.

Two different streams traverse through NAT - signaling and media. A device located

behind a 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

following solutions are provided by the device, listed in priority of the selected method used

by the device:

If configured, uses an external STUN server to assign a NAT address to all interfaces

- see Configuring STUN on page 166.

If configured, uses the single Static NAT IP address for all interfaces - see

''Configuring a Static NAT IP Address for All Interfaces'' on page 167.

If configured, uses the NAT Translation table which configures NAT per interface - see

Configuring NAT Translation per IP Interface on page 167.

If NAT is not configured by any of the above-mentioned methods, the device sends the

packet according to its IP address configured in the Interface table.

The priority list above is applicable only to the Gateway calls.

Mediant 3000