Is it necessary to have different subnets in all 3 offices

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Q: Is it necessary to have different subnets in all the three locations
I have been running windows 2003 Active directory in my office for quite sometime. The DC is also running DNS, DHCP and Exchange 2003. The subnet being used is 192.168.1.x. There is another office location which is already running same platform and same combination of services running on the subnet 192.168.0.x.

Now, we have acquired another office location where we need a new DC (with new domain) but we want to have connectivity of all three offices via VPN over internet. Following are my questions:

1. Is it necessary to have different subnets in all the three locations (as depicted in the diagram attached with this post) ? Someone told me it was necessary because of VPN.

2. How do i create a TRUST relationship between these domains ? is there any article which describes this in detail?

3. Can i create same user id in all of these three domains ? the purpose of this is that i need to move around in all these three locations and i need the ability to authenticate to all domains easily without any configuration change (please mind that the VPN connection between these locations is not permanent, it will be connected and dropped as per needed).

4. Does the DC configuration illustrated in the attached diagram look OK?

I am not a networking expert so i am looking for some expert advice on this.

A:
1. Yes, you different subnets in all the three locations.

2. This page may help, http://www.chicagotech.net/domain%20trusts.htm

3. Yes, you can create the same user and password, especially, they are not reliable connection.

4. The diagram looks OK to me.

Bob Lin, MS-MVP, MCSE & CNE