How to configure DHCP Service on SBS, more specifically, how to hand out subnet mask.

I need help in setting up the DHCP Service on SBS 2000 Server.  Specifically the Scope Option that hands out the subnet mask to the client workstations.   I utilize a class B network (172.25.x.x).  The server hands out the IP addresses okay, but then the work station appears to default the subnet mask to 255.255.0.0.  I would like the DHCP Server to hand the workstation a SM of 255.255.255.0.  I would think this might be a Scope Option, but I cannot determine which one it is.  I see a couple options like "Perform Mask Discovery" and "Mask Supplier Option", but I'm not sure if either of these is what I am looking for.  I would experiment, but I don't want to distrub the 40 users who are presently logged in.   No one is having any trouble, but I can't ping the work stations remotely and I think this is the reason.
Steve MeyerSystem Analyst and DeveloperAsked:
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myin68Commented:
If there's a firewall configured on the PCs it could prevent the ping reply.
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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
I don't believe this is a firewall issue.  But assuming it was, what settings or procedure would you suggest I make to the Windows Firewall on the workstation?
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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
In any case, my original question was, in short, what Scope Option in SBS DHCP Service Settings allows me hand out a specific subnet mask to DHCP clients?
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arnoldCommented:
By defining your scope as a class B network 172.25.x.x you be default define the subnet mask as 255.255.0.0.  Altering the netmask to 255.255.255.0 on the client desktops, will result in a lack of connectivity.  This will happen because the default gateway and the broadcast address wiill not be correct.
i.e. for 172.25.0.2 netmask 255.255.0.0 has a default gateway of 172.25.0.1 with the broadcast address of 172.25.255.255 while for an allocated ip of 172.25.0.2 with netmask 255.255.255.0 the default gateway will remain 172.25.0.1 but the broadcast address will be 172.25.0.255.

An alternate designation for the default gateway and broadcast address might be part of the scope's options, not sure whether the netmask is an option as well.


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EnclosAdminCommented:
What is the actual IP address, Subnet mask and Default Gateway of your SBS server you are setting DHCP up on?
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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
Again, my original question was, what Scope Option in SBS DHCP Service Settings allows me hand out a specific subnet mask to DHCP clients?  To answer this question, one would have to be familure with setting up DHCP services on a Windows 2000 server (or Small Business Server).  What IP address, subnet mask, and GW we have on the server, and what class network we have is imaterial.  Anyone know anything about DHCP Service Scope Options?  To get there in Windows 2000 Server go to <Computer Mangement><Services and Applications><DHCP><Scope><Scope Options>  Right click in right window pane and select <Configure Options>.
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myin68Commented:
I've looked in our DCHP server (2003) and I couldn't find where to set the subnet mask either.  I think the only time you can define your subnet mask is when you first create your scope.  You probably have to delete your current scope, and recreate it with the subnet mask you want.  Your users will get the new settings after they reboot.
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EnclosAdminCommented:
What your IP address is on your DHCP server is relevent meyco.

I use 172.16.XXX.XXX  and my DHCP servers hand out the 255.255.255.0 subnet to all of my clients.  It depends on how your DHCP server's network is set up in the first place which is why I asked.
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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
I have three offices, each with its own DHCP Server.  The offices are 172.25.10.x, 172.25.20.x and 172.25.30.x.  The each handout IP addresses in the range specified 172.25.xx.150-199, were xx is the location 10, 20, 30.   In anycase, the DHCP services always handout subnet 255.255.0.0.  It needs to be 255.255.255.0.   I use Linksys VPN Routers (RV-082) to connect the offices together.  If I statically set IP and SM to 255.255.255.0 on a machine (such as the main server and location 10), everyone can see the machine from any office.  But if I use DHCP, it hands out 255.255.0.0 and that machine can only be seen at it's home location, not by any of the other locations.
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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
Maybe it would be best to just use static IPs for all the work stations; probably would help keep track of them better; but I would still like to figure this out.
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arnoldCommented:
The netmask on the network Interface through which the DHCP service allocates IP addresses must be the same as the clients.

On your DHCP servers, what is the definition of your scope?  Does the scope's definition reflect the 255.255.0.0 netmask? I think because you are using the 172 Private IP space the default netmask is 255.255.0.0. if you do not alter the netmask in the definition of the IP scope, 255.255.0.0 will be allocated as the Netmask via DHCP.  Setting the IPs statically would likely result in connectivity issues.

You should double check your scope's configuration to limit the scope to a Class C (255.255.255.0) even if the IP addresses that are being allocated are on a Class B network.

You could also  remedy this by using the DHCP service on the Linksys VPN routers instead of the Windows DHCP..

Have a look at: http://support.microsoft.com/kb/300429  Refer to the quoted step number 3 below which deals with what you should do if you wish to use a different subnet mask.
3.	Type the range of addresses that can be leased as part of this scope, for example, a starting IP address of 192.168.100.1 to an ending address of 192.168.100.100. Because these addresses are given to clients, they should all be valid addresses for your network and not currently in use. If you want to use a different subnet mask, type the new subnet mask. Click Next.

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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
Arnold, I think you may be the winner of this puzzle.

First, the netmask on the interface is 255.255.255.0.  The DHCP Server hands out IP address, gateway, and DNS, but not netmask.  That is my problem.  Go back and read comment on 02.13.2008 wherein I am trying to figure out how to handout netmask.  Your suggesting that 172 space hands out 255.255.0.0 by default does not gell with the comment posted by EnclosAdmin on 02.18.2008.  I have thought about using the Linksys VPN to hand out DHCP, but I prefer not to decentralize control through the Small Business Server.

H O W E V E R:

All that aside, it appears, after reading KB300429, that the netmask is defined at the time we create the scope in DHCP.  It appears that is only time you can define the netmask.  It appears I cannot change it once the scope is setup.  Now the bonus question; what will happen if I delete the current scope and create a new one.  Will my connected users loose connectivity.  Or should I create a new one first, then delete the old one?  I don't want my phone ringing off the hook when I do this.
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arnoldCommented:
First I do not believe you will be able to create a new scope with a subset of the existing IP space because that would cause a conflict.

The impact to the user will only occur when the IP lease is up for renewal or when the computers are turned on while your DHCP server does not have a configured scope (not running).  

During the change to minimize the impact on the user, you could enable the DHCP service on the VPN router with the IPs outside the Windows SBS space. i.e. if your SBS DHCP server allocates 172.25.20.20-90, you would configure the Linksys VPN to allocate the IPs in the 172.25.20.100-170.  Then delete the scope, enable the DHCP on the Linksys.  Reconfigure the scope on the SBS DHCP server and then disable the DHCP service on the Linksys.

Another option is to increase the lease time to a week and perform the scope change after the current IP  lease  time expires.

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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
Thanks Arnold.
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myin68Commented:
meyco, my last comment "I've looked in our DCHP server (2003) and I couldn't find where to set the subnet mask either.  I think the only time you can define your subnet mask is when you first create your scope.  You probably have to delete your current scope, and recreate it with the subnet mask you want.  Your users will get the new settings after they reboot." is exactly what you said last.  Is that where you got the idea?
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Steve MeyerSystem Analyst and DeveloperAuthor Commented:
I took a chance and just deleted the existing scope while everyone was online.  I created a new scope with subnet mask I wanted and no one ever noticed the change.  Everything is working perfectly now.
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