WARNING, WARNING, Will Robinson ;+)
You misread that article. It is not saying Transmission can only use port 51413.
It is saying that if you choose 51413 (a random port number within the allowed range (1024-65535) not used by anything else) you have to use that same port number in the Transmission configuration and on the router port forwarding rule for external port and internal port.
Because the whole site is oriented to Windoze and really wants to sell you their Windoze based software for setting up rules for the Windoze firewall and Windoze router administration software.
Nooooooooooo.
You have to use the host's NIC MAC address NOT the router's WAN NIC MAC address.
On your host just do
ifconfig
and probably your NIC is assigned to eth0 and from there you can see the MAC address
eth0 Link encap:Ethernet HWaddr NN:NN:NN:NN:NN:NN
where NN are hexadecimal "digits" (0 through 9, or A through F).
If you try the command
arp
you will see the MAC addresses (under the HWaddress column) of neighboring hosts as well.
If you want to check that your NIC is "genuine" or at least has been counterfeited with an appropriate MAC address for that manufacturer, you can go to
and lookup who the manufacturer should be for a particular MAC address.
That is the standard mode of operation for consumer SOHO routers.
If you were manually setting up a firewall on your Ubuntu system using IPTABLES (the basic kernel level tool for creating firewall rules) you would have to have a rule
iptables -t filter -A INPUT -p all -m conntrack --ctstate ESTABLISHED,RELATED -i ${IFACE} -j ACCEPT
As you can see the key words there to allow this functionality to be allowed are for all ports (-p all) on the interface (usually eth0)
ESTABLISHED,RELATED and ACCEPT