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The other roadblock provided by the use of NAT routers is the inability to
accept unsolicited incoming connections from the Internet. In a general
sense, this function acts as a rudimentary firewall and is a net positive for
security, but it does cause headaches for codec users. As shown in Figure
37, a router that receives a connection request doesn’t have a clue where to
forward that stream unless it has specific instructions programmed into it,
known as “port forwarding.” This can work well for fixed installations, but
it’s not always an easy task to obtain that kind of security access on corporate
routers, and forwarding functions are implemented differently on different
hardware. You can easily imagine the complications of obtaining or managing
port forwarding on the LAN at each remote venue, you would certainly
encounter a high volume of grumpy IT staff if you tried.
In describing NAT routing, it’s important to understand the concept of ports.
These are numbers, like the source and destination IP addresses that are
attached to each packet to further qualify which application on a computer
(or codec) is meant to send or receive a packet. In a typical codec application
unit X will send a packet from address A port B to address C port D on the
destination codec Y. A codec that has multiple applications running (like
streaming audio while simultaneously serving a configuration web page)
would deliver these applications from and to different port numbers, but
perhaps to the same IP address. Port numbers are also used by NAT routers
in segmenting applications flowing through them and they may change
source port numbers at will.
Figure 37 - Incoming Packets Hitting NAT Router
