Port forwarding allows remote hosts on the internet to connect to servers on a local network by forwarding incoming connection requests on specific ports to IP addresses and ports of servers on the local network. When a request comes in on a forwarded port, the router checks for a matching forwarding rule and sends the request to the specified internal IP address and port if there is a match, otherwise it drops the request. Port triggers open ports temporarily based on outgoing traffic to allow return connections for things like mail identity information or online gaming.

1.  Port forwarding allows remote hosts from the Internet or outside network
to connect to a server on the local network (inside network)
◦ The default firewall behaviour is to drop any connections starting from the
outside
 Remote hosts typically use protocols such as http, https, ftp, ssh

2.  A remote host sends a request to the public global IP address of the inside
server and the port number of the service required (usually the registered
port no of the destination service e.g. port 80 for http)
◦ If the router is using PAT this is the outside interface address
 The wireless router uses PAT
◦ If the router is using static NAT this is mapped one-to-one address
 The request is routed to the outside interface of the router
 The router checks for a port forwarding rule that matches the protocol and
the destination port of the request
◦ If a match to a rule exists the request is forwarded to the local IP address
and the port number configured in the port forward rule
 Only one inside server can use the global IP/port number mapping
 E.g If there are 2 inside http servers one can use port 80 and one can use port 90
◦ If a match to a rule does not exist the request is dropped

3.  The remote host on the outside network must use the public
global IP address to create the connection
 The port number used in the port forward rule is reserved for
only 1 inside IP address
◦ A port number can’t be used to forward requests to a second server. To
do this, select a different unused port no and create a second port
forwarding rule
 Some applications; multimedia and torrents required several
ports open for connections.
◦ Typically 1 or 2 ports are required for the control channels that setup
and close the connection and monitor the connection.
◦ Extra ports may be required for the data stream, another for
video, another for voice and another for chat
◦ Research the port nos required
◦ Use a port range in the forwarding rule

4.  A port trigger is used to open a port temporarily for a particular
inside host only
◦ The port trigger allows new connections from the outside network to an
inside host that initiated the data flow
◦ E.g. Port 25 is defined as a trigger and 113 as the port
◦ A local host checks mail and triggers port 25. The port trigger allows a
back connection on port 113 (Identity information for mail) to the
originating host
◦ The trigger times out and new connections to 113 are dropped
 The wireless router monitors outgoing traffic for the port numbers
specified in the trigger
◦ When a match is made the IP address of the sending host (local address)
is saved.
◦ When the return traffic arrives back at the router it is forwarded to the
original sending host.
 Port triggers might be used in online gaming