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Multi-Location Subnetting?

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00PS

[H]ard|Gawd
Joined
May 5, 2009
Messages
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As I'm learning more about subnetting I've found myself confused by a specific scenario. Here is what it reads from the Configuring Windows Server 2008 Network Infranstructure self-paced Training Kit:

Suppose you are designing a campus network with 200 hosts spread over four buildings - Voter Hall, Twilight Hall, Monroe Hall and Sunderland Hall. You want each of these four buildings include 50 hosts. If your ISP has allocated to you the /24 network 208.147.66.0, you can use the addresses 208.147.66.1 - 208.147.66.254 for your 200 hosts. However, if these hosts are distributed among four physically separate locations, the distance among them might be too great to allow the hosts to communicate with one another by means of a local network broadcast. By extending the subnet mask to /26 and borrowing two bits from the host ID portion of your address space, you can divide the network into four logical subnets. You can then use a router in a central location to connect the four physical networks.
Click to expand...

Voter Hall
208.147.66.0/26

Twilight Hall
208.147.66.64/26

Monroe Hall
208/147.66.128/26

Sunderland Hall
208.147.66.192/26

All of which share a common router.

------------------------------------------------------------

I for one cannot understand why this makes any difference, and splitting it up would benefit a network how? If it is all on a 255.255.255.192 subnet, it is essentially doing the same thing, right? I guess it does look cleaner/more organised. The only thing I can see useful in this architecture is in a network diagram. Am I missing something?

Bah, after writing this I think it is sinking in...dammit, and that was a lot to copy out of the book.
 
It's literally four different networks. You need to be able to put routes in your routing table that tell the router how to send the packets from one site to another. You need different subnets for this, else your route would have the same origin and destination. That wont work for obvious reasons.

Routing from
208.147.66.0/26
to
208.147.66.64/26

Is very different from routing from
208.147.66.0/24
to
208.147.66.0/24

See how the second example makes no sense? A diagram of the network would probably help you understand this.

You might also create several subnets with VLANs to split up your local network. I prefer to create a subnet and VLAN for each department. It helps narrow down troubleshooting and keeps things organized.
 
Well, I use subnetting for organization mostly. One thing you can do also by having multiple subnets is preventing certain places having any access to a server, etc.

For example you had a server in Voter Hall doing file sharing and you only wanted Twilight Hall to also be able to access it, but no other hall. On the router you can setup a firewall that denies traffic with a destination ip of your Voter Hall server from your Monroe and Sunderland interfaces and allow it from your Twilight interface. If it was all one big subnet then you couldn't block access like that because it'd never hit the firewall because it's in the same subnet.
 
I also wanted to add that the example from the MS book is retarded. Your ISP would never give you one block of IP's for 4 sites. It seems like they wanted to group subnetting and multi-site connections into one example. It's just too much to learn all at once imo.
 
First off, Proto, Casey, thank you. The book definitely feels like it is limited in the information, which in turn left me with more why's then I initially had...it is aimed at winsvr2k8, I can't blame them to much. Both of your examples are perfect and my understanding is much clearer they they are different networks. I think it was that the Network ID was all the same (overlooking the extra 2 bits), so I was thinking that regardless of location, it shouldn't have any effect...I was wrong.

So just to be clear, by creating 4 different subnets:
- It is easier to route by desination (in this example, physical location)
- easier to maintain or have control over
- provides clarity and organization

Correct?
 
You're absolutely correct. I highly recommend that you look into more focused subnetting material. It will dramatically help your understanding of networking. This is from Cisco, but subnetting is universal, so it's a great tutorial for anyone looking to learn some solid networking foundation:

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800a67f5.shtml

And yes, K1llaB, you're right. Though broadcasts are becoming more rare from my experience, so I rarely even think about how I might reduce broadcasts on my networks.
 
Protoform-X said:
I also wanted to add that the example from the MS book is retarded. Your ISP would never give you one block of IP's for 4 sites. It seems like they wanted to group subnetting and multi-site connections into one example. It's just too much to learn all at once imo.
Click to expand...

Yes they will. If you only have one Internet connection you'll usually get a single block of IPs that you must then subnet yourself however you want.
 
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