ZyXEL Communications 2002 Series - Manual (Page 43)

Prestige 2002 Series User’s Guide
Chapter 5 Introduction to VoIP 43
Figure 15 NAT
NAT does not translate IP addresses that are imbedded in the data stream. In order to make
VoIP calls, the Prestige must register its public IP address with a SIP register server. The SIP
register server gets the Prestige’s IP address from inside the SIP data stream and maps it to
your SIP identity. If there is a NAT router between the Prestige and the SIP register server, the
Prestige probably has a private IP address and it will be embedded in the data stream.
5.3.2 NAT Types
There are four types of NAT:
• Full Cone
• Restricted Cone
• Port Restricted Cone
• Symmetric
The first three NAT types use mapping for a private IP address that is independent of the
destination address. Symmetric NAT allocates a new mapping for each different destination IP
address. STUN does not work with symmetric NAT routers (
see “Symmetric NAT” ) or
firewalls.
5.3.2.1 Full Cone NAT
In full cone NAT, all requests from the same private IP address and port are mapped to the
same public IP address and port. Someone on the Internet only needs to know the mapping
scheme in order to send packets to a device behind the NAT router. See the following figure.
Computer A is behind a NAT router and has a private IP address of 10.0.0.3 and is sending and
receiving packets on port 80. It is mapped to the public IP address a.b.c.d and port 30080.
Anyone on the Internet (B and C in this example) can send packets to IP address a.b.c.d and
port 30080 and the NAT router will send them to computer A (at IP address 10.0.0.3 port 80).