User's Manual

Chapter 10 Network Address Translation (NAT) 133
10.5 The ALG Screen
Click Network Setting > NAT > ALG to open the ALG screen. Use this screen to enable and disable
the NAT Application Layer Gateway (ALG) in the Device.
The SIP ALG allows SIP calls to pass through NAT by examining and translating IP addresses
e
mbedded in the data stream. When the Device registers with the SIP register server, the SIP ALG
translates the Device’s private IP address inside the SIP data stream to a public IP address. You do
not need to use STUN or an outbound proxy if you enable the SIP ALG.
Figure 87 Network Setting
> NAT > ALG
The following table describes the fields in this screen.
Table 50 Network Setting > NAT > ALG
LABEL DESCRIPTION
SIP ALG Enable this to make sure SIP (VoIP) works c
orrectly with port-forwarding.
Apply Click this to save your changes back to the Device.
Cancel Click Ca
ncel to restore your previously saved settings.
10.6 Technical Reference
This section provides some technical background information about the topics covered in this
chapter.
10.6.1 NAT Definitions
Inside/outside denotes where a host is located relative to the Device, for example, the computers of
your subscribers are the inside hosts, while the web servers on the Internet are the outside hosts.
Global/local denotes the IP address of a host in a packet as the packet traverses a router, for
e
xample, the local address refers to the IP address of a host when the packet is in the local network,
while the global address refers to the IP address of the host when the same packet is traveling in the
WAN side.