Network Address Translation (Nat) And Voice Over Ip (Voip); Nat Mapping With Sip-Alg Router - Linksys SPA9000 Administration Manual

Configuring the SPA9000 for ITSP Interoperability
UserB
UserA
Internet (WAN)
SPA9000
SIP Proxy with
media proxy enabled

Network Address Translation (NAT) and Voice over IP (VoIP)

NAT is a function that allows multiple devices to share the same public, routable, IP address to
establish connections over the Internet. NAT is present in many broadband access devices to
translate public and private IP addresses. To enable VoIP to co-exist with NAT, some form of NAT
traversal is required.
Some ITSPs provide NAT traversal, but some do not. If your ITSP does not provide NAT traversal,
you have several options.
• "NAT Mapping with SIP-ALG Router, " on page 40
• "NAT Mapping with Session Border Controller, " on page 41
• "Configuring NAT Mapping with a Static IP Address, " on page 41
• "Configuring NAT Mapping with STUN, " on page 42

NAT Mapping with SIP-ALG Router

You can achieve NAT mapping by using a router that has a SIP ALG (Application Layer Gateway).
The Linksys WRV200 router is recommended for this purpose, although any router with a SIP-
ALG can be used. By using a SIP-ALG router, you have more choices in selecting an ITSP.
Linksys Voice System Administration Guide
Network Address Translation (NAT) and Voice over IP (VoIP)
Figure 3-2 SPA9000 as a SIP Proxy for Internet Calls
IP Router (firewall)
Hub/switch
Broadband modem
Interface
UserC
Internet
ISP
ITSP
40