Table of Contents
Advertisement
3.5.3
Special AP & ALG
As a NAT router, the NTC-400 Series Router doesn't allow an active connection request from the outside world while client
hosts on the Intranet may use applications that need more service ports to be allowed for passing through the NAT router.
The "Special AP (application)" feature of the router can get around this problem by allowing certain applications requiring
multiple connections to pass through the NAT feature of the router.
The application-level gateway (ALG) allows customised NAT traversal filters to be plugged into the router to support address
and port translation for certain application layer protocols such as FTP, SIP, RTSP, file transfer in IM applications, etc. In order
for these protocols to work through NAT or a firewall, either the application has to know about an address/port number
combination that allows incoming packets, or the NAT has to monitor the control traffic and open up port mappings (firewall
pinhole) dynamically as required. Legitimate application data can thus be passed through the security checks of the firewall
or NAT that would have otherwise restricted the traffic for not meeting its limited filter criteria.
The Special AP feature allows you to request the router to open pre-defined service ports for incoming packets to pass
through once the trigger port is activated by local hosts. As shown in the diagram below, a special AP rule defines port 554 as
trigger port and 6970-6999 as incoming ports. With this setting, the local user at host 10.0.75.100 can access services located
on the Internet. When you open the application, it will activate the Trigger Port and then incoming data packets from the
remote application server will pass through incoming ports 6970~6999.
3.5.3.1
SIP ALG
The NTC-400 Series Router supports the SIP ALG feature to allow one SIP phone behind the NAT router to call another SIP
phone in the Internet, even if the router executes its NAT mechanism between the Intranet and the Internet. The NAT router
monitors the control traffic and opens up port mappings (firewall pinhole) dynamically to know about an address/port
128 of 359
© NetComm Wireless 2019
Figure 152 – Special AP List
Figure 153 – Special AP feature
User Guide
Advertisement
Table of Contents
