Secondly, there is an assumption (that in this case), the avionics actually talk IP at all
IFE does, have a look at this video on youtube
http://youtu.be/F1-rfMBp6vw
i386 computer running Redhat Linux 2.4.10, starts IP services, including ICMP, UDP, TCPIP, IGMP, and connects with port 50071. It uses Iptables as well, so there is more than enough information to understand how they have set it up. As another poster pointed out, you can buy one of these computers off ebay for $35.
No hacking or sniffing, no hardware, not even on the aircraft, just watching a youtube video.
How do I get it off FlightRadar24?
What is seen on FlightRadar24 comes from two feeds, one from ATC (in some parts of the world), and the other from people with a receiver and a computer connected to the internet that sends the observed positions back to a centralized network. You can build a receiver that will obtain the ADS-B position as well as ACARS messages from aircraft very cheaply, all you need is a USB TV tuner and an antenna, and run free software on your PC. That is not how IFE systems work, they obtain position, wind, altitude, outside air temperature within the aircraft. IFE also has ground based, and space based communication links from the aircraft. Space based communications needs to know the exact position so it knows where to point the antenna, i.e. MH370 antenna pings.