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this issue and have taken steps to relieve the pain. If your router supports a
SIP Application Layer Gateway (ALG), then enabling this option can fix the
issue. Essentially, the router will get smart enough to read your SIP hand-
shake, find the outgoing "address" field, and replace it with your public IP.
This is a pretty slick solution, but there may be environments when you
are not aware whether this option is supported on your router, or have the
ability to enable it. So on to solution two;
STUNning Success
Another technique for working around the SIP-Router issue is by using a
protocol called STUN. This can be enabled in Comrex codecs, and essen-
tially allows for the codec to learn what its public IP address is. It does this
by contacting a STUN server out on the Internet (the default one is main-
tained by Comrex) and simply asking. If this option is enabled, the codec
itself will handle the "address" switcheroo.
Be aware of the dreaded "battling workarounds" issue. In our simple de-
scription, we left out the fact that ports are being translated by the router
as well as IP addresses. If the ALG-enabled router receives an unexpected
result in the SIP "address" field (as it might if using STUN), it may not trans-
late ports as expected, and it's likely that the call will fail. When in doubt,
the best technique is to try a SIP call with STUN turned off, and if the re-
turn channel fails, try enabling STUN.
Fix of Last Resort
Finally, there's a brute-force option available on Comrex Codecs when
STUN ports are blocked by a firewall, or it can't be used for some other
reason. Under Advanced System Settings, a field is available called "Public
IP Override". Any address put into that field will be pasted into the "ad-
dress" SIP field. So if you know what your public IP address is (can be
gotten from many websites via a browser) you can manually paste it here.
Keep in mind, this is often subject to change over time (and obviously if
you use a different network) so it's important to remember this change has
been made on your codec.