User's Manual Part 1

Chapter 7 Network Setting
BM2022w User’s Guide
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192.168.1.35 to a third (C in the example). You assign the LAN IP addresses and the ISP assigns
the WAN IP address. The NAT network appears as a single host on the Internet.
Figure 44 Multiple Servers Behind NAT Example
Trigger Ports
Some services use a dedicated range of ports on the client side and a dedicated range of ports on
the server side. With regular port forwarding you set a forwarding port in NAT to forward a service
(coming in from the server on the WAN) to the IP address of a computer on the client side (LAN).
The problem is that port forwarding only forwards a service to a single LAN IP address. In order to
use the same service on a different LAN computer, you have to manually replace the LAN
computer's IP address in the forwarding port with another LAN computer's IP address,
Trigger port forwarding solves this problem by allowing computers on the LAN to dynamically take
turns using the service. The BM2022w records the IP address of a LAN computer that sends traffic
to the WAN to request a service with a specific port number and protocol (a "trigger" port). When
the BM2022w's WAN port receives a response with a specific port number and protocol ("incoming"
port), the BM2022w forwards the traffic to the LAN IP address of the computer that sent the
request. After that computer’s connection for that service closes, another computer on the LAN can
use the service in the same manner. This way you do not need to configure a new IP address each
time you want a different LAN computer to use the application.
ALG
Some applications, such as SIP, cannot operate through NAT (are NAT un-friendly) because they
embed IP addresses and port numbers in their packets’ data payload. Some NAT routers may
include a SIP Application Layer Gateway (ALG). An Application Layer Gateway (ALG) manages a
specific protocol (such as SIP, H.323 or FTP) at the application layer.
A SIP ALG allows SIP calls to pass through NAT by examining and translating IP addresses
embedded in the data stream.
UPnP
Universal Plug and Play (UPnP) is a distributed, open networking standard that uses TCP/IP for
simple peer-to-peer network connectivity between devices. A UPnP device can dynamically join a
network, obtain an IP address, convey its capabilities and learn about other devices on the network.
In turn, a device can leave a network smoothly and automatically when it is no longer in use.
How do I know if I'm using UPnP?