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1.1 Information about IP Telephony Service
Therefore, every time a domain name is used, a DNS server must translate the name into the corresponding
IP address, and vice versa. For example, the domain name www.example.com may be translated to
192.0.34.166. If one DNS server does not know how to translate a particular domain name, it asks another
one, and so on, until the correct IP address is returned.
NAT (Network Address Translation) Traversal
When NAT/NAPT (Network Address Port Translation) is enabled, the router translates a local IP address from
the SIP-GW4 card into a global IP address. However, the router with NAT enabled does not translate local IP
addresses stored in SIP messages into global IP addresses.
Therefore, the address which the SIP Server recognizes as the destination IP address to reply to is actually
the local IP address of the SIP-GW4 card, not the global IP address of the router. Therefore, if the SIP server
receives a SIP message from the SIP-GW4 card and sends a message back to the SIP-GW4 card using the
address stored in the SIP message, the packet information will not reach the SIP-GW4 card.
STUN Servers function to solve the global IP address problem under certain NAT conditions, for example, in
case of full duplex communication. A STUN Server, used alongside the SIP Server, finds out the global IP
address of the router with NAT enabled. With the STUN feature enabled, the packet information sent by the
SIP Server is able to "traverse" NAT and reach the SIP-GW4 card.
The settings can be configured to specify whether to enable the NAT Traversal feature for each ISP/ITSP. In
addition, the NAT Traversal method can be selected from "STUN" and "Fixed IP Address" (refer to 3.4.3 NAT
Traversal).
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Installation Manual
Document Version 2008-10
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