User guide

Advanced Router Settings
62
Verizon 4G LTE Broadband Router
Port Triggering to Open Incoming Ports
Some application servers (such as FTP and IRC servers) send replies to multiple port
numbers. Using the port triggering function of your router, you can tell your router to open
more incoming ports when a particular outgoing port originates a session.
An example is Internet Relay Chat (IRC), outlined in the process below:
1. Your computer connects to an IRC server at destination port 6667.
2. The IRC server responds to your originating source port, and sends an “identify” message to
your computer on port 113.
3. Using port triggering, tell the router, “When you initiate a session with destination port 6667,
you have to allow incoming traffic also on port 1
13 to reach the originating computer.”
After your have defined a port triggering rule, the process for IRC expands:
1. You open an IRC client program to start a chat session on your computer.
2. Your IRC client composes a request message to an IRC server using a destination port
number of 6667, the standard port number for an IRC server process, and sends this
request message to your router.
3. Your router then:
a. creates an entry in its internal session table describing this communication session
between your computer and the IRC server;
b. stores the original information;
c. performs Network
Address
Translation (NAT) on the source address and port; and
d. sends this request message through the Internet to the IRC server.
4. Noting your port triggering rule and having observed the destination port number of 6667,
your router creates an additional session entry to send any incoming port 113 traf
fic to your
computer.
5. The IRC server sends a return message to your router using the NAT
-assigned source port
(for example, port 33333) as the destination port. The IRC server also sends an identify
message to your router with destination port 113.
6. Upon receiving the incoming message to destination port 33333, your router checks its
session table to determine whether an active session for port number 33333 exists. Finding
an active session, the router restores the original address information that NAT replaced and
sends this reply message to your computer
.
7. Upon receiving the incoming message to destination port 113, your router checks its session
table and learns that an active session for port 1
13 exists and is associated with your
computer. The router replaces the destination IP address of the message with the IP
address of your computer and forwards the message to your computer.
8. When you finish your chat session, your router eventually senses a period of inactivity in the
communications. The router then removes the session information from its session table
and no longer accepts incoming traffic on ports 33333 or 1
13.
To configure port triggering, you need to know which inbound ports the application needs and
the number of the outbound port triggering the opening of the inbound ports. Y
ou can usually
get this information from the publisher of the application or user groups or newsgroups.