Windows Forest


I am setting up a replacement forest/domain for one of my sites with approx 130 users.  I want to setup the new forest/domain and get everything setup correctly, while our 130 users are still connecting to the old forest/domain.  Can I do this while they are still using the existing IP Subnets?  e.g. for both the old forest and the new forest?  I won't be setting up DHCP until the weekend that I am migrating all the users, so this won't be a problem.  

Is there anything else that may cause problems or am I ok to proceed?

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Jeff GloverSr. Systems AdministratorCommented:
Yes, you can have both Forests/Domains using the same subnet. No problem there. DNS is the only thing you need to make sure is correct. If your new Forest/Domain is using AD integrated DNS, then make sure the old DNS server has a Conditional Forwarder to the new domain.( and vice versa) This makes the change over easier.

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minniejpAuthor Commented:
We don't want anything from the old domain (it was a bit of a mess and we don't want anything from it) that still ok?
Jeff GloverSr. Systems AdministratorCommented:
Sure. Domain (and Forests) are basically only logical and security boundaries. I have a production domain and 2 test domains sharing the same subnet. No problems. You setup your new forest/domain and get it the way you want. DNS setup, etc.... The reason I say to put conditional forwarders in DNS is so when you decide to swap users and PCs from one domain to the other, you are not so dependent on DHCP being perfect to do it. If you are going to move DHCP, you can backup and restore the DHCP database to the new server or you can just set the new one not to give addresses that are already assigned. When you start the new DHCP server, you have to actually stop the DHCP server service on the old one or the clients will not move to the new one gracefully.
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minniejpAuthor Commented:
great! many thanks
minniejpAuthor Commented:
is there anything else that I should be looking out for?
Jeff GloverSr. Systems AdministratorCommented:
Not really. It is hard to cover all the bases just answering posts but make sure your existing PCs can ping your new domain before you try to move them over. I would say look into ADMT to move the computers and maybe even users but that is your choice. Here is a link to ADMT style restructuring
minniejpAuthor Commented:
Will SzymkowskiSenior Solution ArchitectCommented:
You can have multiple Forest/domains on a single subnet, however, when you have this scenario you will need to configure DHCP for each respective Forest to exclude the others range from being handed out.


Forest/Domain 1
IP Scheme -
Exclude - (from dhcp)

Forest/Domain 2
IP Scheme - -
Exclude - (from dhcp)

However, it is best to have them on separate vlans if possible. In order for no issues to happen you need to make sure that the DHCP is set correctly and also make sure that no leases from the upper range "" (that is from the other domain) is not being leased out by any of your clients already.

minniejpAuthor Commented:
What if I don't install the DHCP role until the weekend I will be doing the changeover (when no users are on the network)? and just give the new domains static ips?
Will SzymkowskiSenior Solution ArchitectCommented:
and just give the new domains static ips?

You can do this however, you will still need to exclude the IP's that you use for the other forest in the DHCP management console of the original forest.

You need to do this because you are sharing the same subnet and all IP's need to be accounted for.

Aside from that it should be fine.

Jeff GloverSr. Systems AdministratorCommented:
You should have had some IP addresses excluded for your Servers anyway. Personally, I would change the Lease length for the current DHCP scope to 4 hours or so. It helps. It will cause some extra traffic for a bit but doing this makes changing DHCP over easier. Since you said you have 130 clients and are using a Class C, you cannot exclude half of your addresses and accommodate all of them. (130 + 130 = 260. A class C is 255 only.  If you plan to do this on the weekend, when no users are there, you can shutoff DHCP server service on the old server a few hours before the changeover and the clients will lose their address. Then you can Turn on the new one and the clients should get new addresses in the new Database. Otherwise, you can simply migrate DHCP to the new server. There are pretty good instructions here
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Windows Server 2008

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