ZyXEL Communications 91-009-073003B ユーザーズマニュアル

Chapter 19 ALG

ZyWALL USG 50 User’s Guide

The ZyWALL can function as an Application Layer Gateway (ALG) to allow certain 
NAT un-friendly applications (such as SIP) to operate properly through the 
ZyWALL’s NAT and firewall. The ZyWALL dynamically creates an implicit NAT 
session and firewall session for the application’s traffic from the WAN to the LAN. 
The ALG on the ZyWALL supports all of the ZyWALL’s NAT mapping types.

The FTP ALG allows TCP packets with a specified port destination to pass through. 
If the FTP server is located on the LAN, you must also configure NAT (port 
forwarding) and firewall rules if you want to allow access to the server from the 
WAN.

• The H.323 ALG supports peer-to-peer H.323 calls.
• The H.323 ALG handles H.323 calls that go through NAT or that the ZyWALL 

routes. You can also make other H.323 calls that do not go through NAT or 

routing. Examples would be calls between LAN IP addresses that are on the 

same subnet.

• The H.323 ALG allows calls to go out through NAT. For example, you could make 

a call from a private IP address on the LAN to a peer device on the WAN. 

• The H.323 ALG operates on TCP packets with a specified port destination.
• The ZyWALL allows H.323 audio connections. 
• The ZyWALL can also apply bandwidth management to traffic that goes through 

the H.323 ALG.

The following example shows H.323 signaling (1) and audio (2) sessions between 
H.323 devices A and B.

Figure 201   H.323 ALG Example 

• SIP phones can be in any zone (including LAN, DMZ, WAN), and the SIP server 

and SIP clients can be in the same network or different networks.