Alg; Chapter 20 Alg; Alg Overview; What You Can Do In This Chapter - ZyXEL Communications ISG50-ISDN User Manual

Integrated service gateway

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Application Layer Gateway (ALG) allows the following applications to operate properly through the

ISG50's NAT.

• H.323 - A teleconferencing protocol suite that provides audio, data and video conferencing.

• FTP - File Transfer Protocol - an Internet file transfer service.

The ALG feature is only needed for traffic that goes through the ISG50's NAT.

Use the ALG screen

Application Layer Gateway (ALG), NAT and Firewall

The ISG50 can function as an Application Layer Gateway (ALG) to allow certain NAT un-friendly

applications (such as H.323) to operate properly through the ISG50's NAT and firewall. The ISG50

dynamically creates an implicit NAT session and firewall session for the application's traffic from the

WAN to the LAN. The ALG on the ISG50 supports all of the ISG50'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 ISG50 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 ISG50 allows H.323 audio connections.

• The ISG50 can also apply bandwidth management to traffic that goes through the H.323 ALG.

ISG50 User's Guide

(Section 20.2 on page

338) to set up H.323 and FTP ALG settings.

ALG