Chapter 12 Network Address Translation (NAT)
VSG1435-B101 Series User’s Guide
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12.6 The ALG Screen
Some NAT routers may include a SIP Application Layer Gateway (ALG). A SIP ALG
allows SIP calls to pass through NAT by examining and translating IP addresses
embedded in the data stream. When the ZyXEL Device registers with the SIP
register server, the SIP ALG translates the ZyXEL 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 your ZyXEL Device is behind a SIP ALG.
Use this screen to enable and disable the SIP (VoIP) ALG in the ZyXEL Device. To
access this screen, click Network Settings > NAT > ALG.
Figure 72 Network Settings > NAT > ALG
The following table describes the fields in this screen.
12.7 The Sessions Screen
Use the Sessions screen to limit the number of concurrent NAT sessions all clients
can use.
Table 49 Network Settings > NAT > ALG
LABEL DESCRIPTION
ALG Enable this to make sure applications such as FTP and file transfer in IM
applications work correctly with port-forwarding and address-mapping
rules.
SIP ALG Enable this to make sure SIP (VoIP) works correctly with port-
forwarding and address-mapping rules.
Apply Click Apply to save your changes.
Cancel Click Cancel to restore your previously saved settings.
