D-Link DFL-600 - Manual (Page 51)

Application Gateway (ALG)
Some applications (programs running on a PC on your LAN) require multiple
TCP or UDP ports to function properly. Applications such as Internet
gaming, video conferencing, and Internet telephony are some examples of
applications that often require multiple connections. These applications often
conflict with NAT, and therefore require special handling. The Special
Applications page allows you to configure your DFL-600 to allow computers
on your LAN to access servers on the WAN that require multiple TCP or
UDP connections.
Application Name This is a reference − usually the name of the
application. In the above example, Netmeeting
is the application, and this is used to name this
entry.
Trigger Port Range This is the TCP or UDP port range used to
trigger, or start, the application. It can be a