Network Address Translation Support; Device Located Behind Nat - AudioCodes Mediant 800B User Manual

Note: If a static route is required to access OAMP applications (for remote
management, for example) and the route is not configured correctly, the route is not
added and the device is not accessible remotely. To restore connectivity, the device
must be accessed locally from the OAMP subnet and the required routes be
configured.
12.7

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.

12.7.1 Device Located behind NAT

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 device
provides the following solutions, listed in priority of the method used by the device:
1. (Gateway Application Only) If configured, uses the single Static NAT IP address for all
interfaces - see Configuring a Static NAT IP Address for All Interfaces on page 147.
2. If configured, uses the NAT Translation table which configures NAT per IP network
interface - see Configuring NAT Translation per IP Interface on page 148.
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 IP Interfaces table.
Note: The priority list above is applicable only to Gateway calls.
User's Manual
Mediant 800B Gateway & E-SBC
146 Document #: LTRT-10298