[Asterisk-Users] Asterisk, SIP, and Registrations

[Steve Woolley]

I am very new to asterisk and SIP so these questions may be fairly
basic.
I have setup an asterisk system based on Red Hat 8.0 Linux with a few
analog cards from Digium (X101P's). I have also purchased a SNOM 200
phone and a couple of soft SIP clients for windows. Everything (so far)
is working nicely.

As I understand it, the asterisk server performs as a SIP proxy. The
sip.conf file sets up  the SIP user agents (clients) connections --
whether they are dynamically addressed, are they "friendly", etc. The
name given in the [context] is what is used by the extensions.conf file
to associate an extension with this user agent.

 Example:
   ;
   ; SIP Configuration for Asterisk
   ;
   [general]
   port = 5060                     ; Port to bind to
   bindaddr = 172.16.1.155         ; Address to bind to
   context = default               ; Default for incoming calls
   ;
   [ext1111]
   type=friend
   secret=1234
   host=dynamic
   defaultip=172.16.14.1

   ;
   ; excerpt from extensions.conf for Asterisk
   ;    
   exten => 1111,1,Dial,SIP/ext1111|20
   exten => 1111,2,Voicemail,u1111
   exten => 1111,102,Voicemail,b1111

Now based on my ability to configure the user agent(s), which is
sometimes difficult because it seems every user agent vendor seems to
have a slightly different SIP vocabulary and sparse documentation, I
point my SIP clients at asterisk, dial the extension, and it works.

Here are my questions:

1) What purpose does the "defaultip" entry in the sip.conf file serve if
it is set to host=dynamic?


2) If my asterisk server has just one IP address, can I use bindaddr =
0.0.0.0 instead of specifically setting the IP address?

3) I assume the use of username and password in the sip.conf is strictly
for the ability to authenticate a user agent to the asterisk server --
so some yahoo just doesn't stick any old user agent on your network?

4) The BIG question: it is my understanding of SIP that the really cool
feature (of SIP) is the ability to register a user such as
(sip:swoolley at adstelecom) at the user agent level and some type of
registration service would associate sip:swoolley at adstelecom with the
user agent (example: 1111 at 172.16.14.1). All the SIP clients I have dealt
with have a line for "register as" with username and password. 

  Can asterisk as a registration server?

  I noticed "sip show registrations" command inside asterisk, but no
entries ever show up?

  If asterisk can act as a registration server, how is the
feature/service started and configured?

  
Thanks for some insight.


Steve Woolley
swoolley at adstelecom.com