What is the Subnets folder for in Active Directory Sites and Services?

In Win 2003 domain Active Directory Sites and Services, under the Sites there is Subnets folder. Is it required for me to put in all the subnets IP we have? If I don't that specific subnet will not work? Or that is just for information purpose? please see the attachment.
CastlewoodAsked:
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Ben HartCommented:
Yes you want to enter your subnets in there, then you associate the subnet to the site.

https://technet.microsoft.com/en-us/library/cc754697.aspx

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Benjamin VoglarIT ProCommented:
yo_beeDirector of Information TechnologyCommented:
If you have multiple sites with services at each site that you want your clients to communicate  with before call out to any other server in another site then you only assign the subnets you want to communicate with that site.  If you do not create a subnet in the subnet folder and it exists in your environment then the clients will do a round-robin to the first server that responds. So you should add all your subnets to the subnet folder in S & S then assign them to the proper sites

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So if you have a NYC, DC and London site. Each site has a DC and DFS File server.
NYC = 192.168.9.1/24, 192.168.10.1/24 , 192.168.11.1/24
DC = 192.168.2.1/24
London = 192.168.3.1/24

You would assign each one of your sites with the appropriate subnet.  By assigning the correct subnet your clients would not traverse the WAN to get the files or authenticate.

This helps  reduce the latency as well as necessary  traffic.

Is you don't have severs in other sites and all clients call back to a data center then subnets do not need to be setup nor do you have to do anything with the default setup of Sites and Services.
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CastlewoodAuthor Commented:
Thanks yo_bee for the clear explanation. One thing keeps puzzling me....
In an environment with multiple subnets, where are the default gateways defined? and also how to make all different subnets can talk to each other?

For instance in your example:
For the subnet of
192.168.9.1/24, the DW is 192.168.9.1
192.168.10.1/24 , the DW is 192.168.10.1
192.168.11.1/24 , the DW is 192.168.11.1
192.168.2.1/24, the DW is 192.168.2.1
192.168.3.1/24, the DW is 192.168.3.1

Where are those Default Gateways defined? I mean when you use "ping -a" to ping the DW's IPs they will successfully reply but without the host name. As the result I can not know their host name and don't know where they are. I'm wondering where are they defined. On a router? or a server? Can you help?
yo_beeDirector of Information TechnologyCommented:
If you have different subnets, by the nature of the setup they are isolated from each other unless you create a bridge between the two and there also has to be a trust between them so communication can move freely.  This can be achieved by multiple NICs on a server each on a different subnet and multiple home each card.  You can also achieve this with managed switches and routers.

Are you able to ping 192.168.2.1/24 from 192.168.10.1/24?
If yes then you have the subnets bridged.  Most likely your network is a bunch of vLans  

Since you posted /24 that means you have 254 total addresses that are available and each one of the subnets with a /24 has a DW .1 since the first 3 octets of the IP-Address are fully used up.  If you had a /23 that means you have a single bit available in the 3rd octet and the entire 4th octet. This gives you 192.168.1.1 - 192.168.2.254  with a DW 192.168.1.1 since this is the first IP-Address  in the list. This can also look like 172.16.129.1 -172.16.130.254, but if it was a /24 then you only have 192.168.1.1 – 192.168.1.254 or 172.16.129.1 -172.16.129.254  and the DW would be either 192.168.1.1 or 172.16.129.1

The more bits you use the more subnets you have and the less guest that can be part of the network.
So if you have a /25 you just barrowed a bit for the 4th octet cutting your guess in half from 254 to 128.  This means that 1 -127 and 128-254 are on two separate networks and the DW’s would be 192.168.1.1 and 192.168.1.128

All of this is dictated by the laws of network and are purely configured on the devices.
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