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We're adding an additional Exchange server at another physical site whose subnet (2.2.2.x) currently exists in a different AD site (SiteB) than our mailbox server site (SiteA). We cannot eliminate the AD SiteB, so we were thinking of splitting the AD subnet into different smaller AD subnets. We'd likely split them into .0-239 and .240-245 or so, then move the smaller subnet into SiteA as most of the in use addresses would fall into the former range. I noticed that you can create overlapping subnets so the plan would be to create the new subnets and leave them in the current site, move the smaller 240-245 subnet to SiteA, then delete the /24 subnet. Is this sound and what effect if any would this have on existing machines? Thanks.

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What problem are you trying to solve? – tomjedrz Nov 13 '09 at 18:16
The AD site topology really should reflect the reality of the physical network. Even if there isn't any impact to replication or message routing by this change I would make the AD site topology reflect reality, just in case it matters in the future (new DC deployments into those subnets, etc). – Evan Anderson Nov 13 '09 at 18:20
Thanks for your comments. The problem we're addressing is ensuring that the Exchange servers are all in the same AD site. For example, the CAS server cannot serve up mailboxes on a different AD site via OWA. Physically, the physical sites are connected by site-to-site VPNs over a 100MB line. Ideally we would clean up the AD sites but because there are other domains involved, we thought this best. – Ryan Migita Nov 13 '09 at 18:49
up vote 0 down vote accepted

AD will match the most specific subnet object. "240-245" isn't something you can chunk up into any sort of valid subnet object.

What you need to do here is define /32 subnet objects on a per IP basis. My suggestion is that you leave the existing subnet object linked to SiteB, and then create /32 objects for 240-245 in SiteA.

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I'm not aware of any problems with what you're looking to do. You want your AD site topology to describe your physical network topology accurately.

If domain controller computers will be moving to a new site you'll need to move their computer objects in the "Active Directory Sites and Services" console. Client computers and member servers will be unaffected.

Exchange 2003 will be unaffected.

Exchange 2007 routes inter-server traffic based on AD site topology. If you're leaving the Exchange Server computer's location alone there will be no effect.

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