Chapter 12 Network Address Translation (NAT)
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 71 Network Settings > NAT > ALG
The following table describes the fields in this screen.
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 |
| rules. |
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SIP ALG | Enable this to make sure SIP (VoIP) works correctly with port- |
| forwarding and |
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Apply | Click Apply to save your changes. |
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Cancel | Click Cancel to restore your previously saved settings. |
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12.7 The Sessions Screen
Use the Sessions screen to limit the number of concurrent NAT sessions all clients can use.
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