Need help to setup asterisk inside firewall protected LAN

Hi I am new to asterisk.I am trying to configure Asterisk-1.4.9 inside a Firewall protected LAN(Asymmetric NAT) only for internal communication inside the office/LAN,but without any luck so far.My aim is to connect all the users inside my office,who can talk to each other,not outside world for the time being.

I want a very basic setup to connect all the users inside offfice/LAN.Can anyone provide me detailed steps how to configure extensions.conf & sip.conf etc file in firewall scenario.

Also i want to inform that my ASTERISK server and sjphone/SIP clietns are all inside same LAN & my asterisk server has static ip but it doesn’t have a public domain name/public IP. Is it necessary to have a public IP for the asterisk server?

Can anyone put some light on
1.What is outbound sip proxy? why it is needed?
2.what is ougoing sip provider & DID?
3.Do i need to install asterisk on a public ip server and keep it outside firewall?
4.What is externip actualy,can i put my asterisk server’s static ip address there?
5.If it is not static how can i configure it as static?