Comrex BRIC-Link II, Product Manual

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This can work well for fi xed installa ons, but it’s not always an easy task to obtain that kind of security access on
corporate routers. Addi onally, forwarding func ons are implemented di ff erently depending on the hardware. One
can easily imagine the complica ons of obtaining or managing port forwarding on the LAN when arriving at a new
remote venue. This would likely encounter a large amount of resistance or confusion on the part of local IT sta
ff .
In describing NAT rou ng, it’s important to understand the concept of ports. These are numbers, like the source and
des na on IP addresses, that are a ached to each packet. They further qualify which applica on on a computer
(or codec) is meant to send or receive a packet. In a typical codec applica on, Codec X will send a packet from
Address A/Port B, to Address C/Port D on the Des na on Codec Y. A codec that has mul ple applica ons running
(like streaming audio while simultaneously serving a con
fi gura on web page) would deliver these applica ons from,
and to, di ff erent port numbers, but perhaps to the same IP address. Port numbers are also used by NAT routers in
segmen ng applica ons fl owing through them, and they may change source port numbers at will .
The term “Network Address Transla on” ( NAT ) refers to the ability of a router to translate requests from computers
(or codecs) within its LAN into formats usable on the public internet. On its most basic level, this involves replacing
the private “source” or return IP address in each packet with the true public IP and remembering where that packet
was sent. This insures that any response can be forwarded back to the proper device.
A good way to think of this is that an outgoing packet “punches a hole” in the router, through which authorized
reply packets may be returned to the codec for a limited me (as shown in
Figure 58 ).
Switchboard aids in breaking through these di ff erent types of routers for incoming calls. Because it is in constant
contact with all subscribed codecs, it can send and receive test pa erns to determine whether one or more NAT
routers exist on a link and what type they are. It can then choose a connec on method to be used to circumvent
any issues.
Depending on the what network restric ons it detects, Switchboard can:
• Instruct the calling codec to make a normal connec on (no NAT detected).
• Use the hole punched by connec on to the Directory Server for incoming connec ons from other codecs.
• —or instruct the called codec to make the connec on in the reverse direc on.