[Asterisk-Users] A configuration question

[hugolivude]

I'm setting up an Asterisk server with 2 FXO & 2 FXS ports on a
TDM400.  All calls will come in and out through PSTN (no external
VoIP) on the FXO/FXS ports.  The Asterisk server will serve 15 users
each using a soft-phone.  Some users are mobile, but most are local
using desktops.  My question concerns registration of the soft-phones.

I could have the soft-phones register directly to the Asterisk server
using its internal IP address (192.168…) or I could have them register
to our external, static IP WAN address (I have the proper ports
forwarded to the Asterisk machine).

Obviously the mobile machines will have to register to the WAN IP
address, but what about the others?  Ideally I'd like to set them all
up the same way, but I'm concerned about a bandwidth bottleneck onto
the WAN from the local machines if they register via the external IP
address.

I don't think that the SIP signalling will create a bandwidth issue if
the local machines register to the external WAN address, but once a
call is established there could be a bottleneck depending upon how the
routing works.

If the local machines register to the external WAN IP address, will
the voice traffic be routed out onto the WAN and then back in, thus
creating a bottleneck or can I configure it so that Asterisk knows
that some of the clients registered to it through the WAN IP address
are actually local machines?  This way local voice traffic would go
directly between Asterisk and the local machine thus avoiding a WAN
bottleneck.  If this were possible, how would I do it?

A second question as well, which may be related.  I'm not clear on how
the "proxy server" field is used on the soft-phone – for example the
X-Lite.  In my configuration, I'll be using the same IP address in
both the "Domain/Realm" and "SIP proxy" fields (unless of course I
hear otherwise).  Under what circumstances would you use a different
IP address for the "Domain/Realm" and "SIP Proxy" servers?

Thanks,
Hugh