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I will be migrating my Windows 2008 Domain from a /24 to a /16 network (as an example only, not intended to be too specific), and the process appears to be poorly documented from my searching. I understand that I will need to change any static IP addressing and modify DNS (reverse lookup zones), DHCP (ranges) and Active Directory Sites and Services (site IP range) accordingly, however I am concerned that there could be some gotchas that I'm not aware of.

Links to any resources or any advice on migrating Windows Domains to new networks would be much appreciated. I will also be more than happy to post my own experiences when the migration has been completed to help future visitors.

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up vote 3 down vote accepted

That's an awfully big broadcast domain. Why don't you get an L3 switch and make multiple /24s and route between them. That will cut down on broadcast traffic while making it so that you don't have to change anything on the existing servers.

Edit: Upon further review, I haven't really answered the question. While it's not a good idea to do this in your particular situation, the same idea might be valuable for someone expanding a large server subnet from /24 to /23 or something like that.

You should be fine doing this. Configure it on your router/L3 switch first, then your DCs, then the DHCP scopes and AD Site definitions, then modify or create an appropriate reverse lookup zone, then your member servers. You don't need to change anything other than the network settings on each computer/server.

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Hi MDMarra. We're on a gigabit network, but we don't intend to come anywhere near occupying the entire subnet - we'd just like to avoid having to do this again between now and when we migrate to multiple routed subnets whilst keeping complexity to a minimum in the meantime. We're also not flush with cash at the moment, having just recently heavily invested in new server infrastructure. – Ehtyar Mar 15 '12 at 10:59
You don't need to be flush with cash. It just takes 1 L3 switch. Why bother going through all of this just to redo it down the roads. It just seems like a poor decision, IMO – MDMarra Mar 15 '12 at 11:06
+1, Don't go to /16 if you don't intend to use it, otherwise you will end up using it. Stick with /24 and use L3 switches, as MDMarra points out, you can get L3 switches for peanuts these days. – Bryan Mar 15 '12 at 11:31
@MDMarra I appreciate your answering my question regardless of your views on the subject. In your opinion (@Bryan please feel free to weigh in also), what is the largest subnet acceptable for a Windows(60%)/OS X(20%)/VOIP phone(20%) environment? – Ehtyar Mar 15 '12 at 22:41
I generally don't like anything larger than a /22. Imagine that something causes a broadcast storm and knocks a whole subnet offline while you fix it. If everything s on the same large subnet, everything is down. This is why my servers always have their own subnet and I like individual /24 or /23s for each of our buildings. Obviously do whatever makes sense for you. – MDMarra Mar 15 '12 at 22:45

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