Allow extension registration only from specific IP address

Hi,

I am running the Trixbox 2.6.0.7 on my local network, and I'm using it only for local calls. Now I want to make an extension for every host on the network (about 200). Now I want to set that one extension can register to server only from specific IP address. That is because I will put the same password to all hosts, and the extension number will be unique (last four digits of an IP address of the host). Every host have static private IP address. Than users can use the service if they want without bothering me to make them account. There will be an extension waiting on server for every user that want to use the service, and I will write only one tutorial on how to setup the x-lite. I tried to change host field on extension setup from dynamic to an ip address of the host, but then the phone couldn't register at all. What should I do?

Thanks in advance