Question

Setting a correct subnet mask in windows XP

Asked by: fissionman1

Hello,
   I'm the administrator of a computer lab of about 15 computers running Windows XP Pro. My lab got new computers recently and I'm attempting to set them up with Altiris deployment solutions software, which can image and deploy computer images over a network.
   My problem is this: I have a server that can deploy computer images to any number of computers, However they need to be in the same workgroup and visible to each other. Most of my computers have IP addresses in the range of 128.104.202.XX and are automatically assigned with a subnet mask of 255.255.255.0. The server and several other computers have a different range of IP addresses, ie 128.104.17X.XXX, and a different subnet mask (255.255.252.0). All of these are automatically network settings are automatically assigned by my companies DHCP server. Each range seems to have it's own workgroup, even though they all share the same workgroup name.
  The only way I've been able to make this work is to manual change all of the subnet
masks to 255.255.0.0, which forces the computers to have static IP addresses. However I've been told that static IP addresses are not allowed on the network. I've brought this situation up to my companies network admins and I've been told

"All of your devices are on the same subnet.  You will just need to
figure out how to set your subnet mask to capture it.  Yours is a bit
too extreme."

 So I guess my question is, is there any way I could make this work? Is there a way
to change the subnet mask without setting a static IP. At this point I'm guessing that having the subnet information, or the assigned IP addresses,  changed on the DHCP server isn't going to happen. Any help would be appreciated.

Thanks

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Asked On
2009-08-12 at 13:53:52ID24647985
Tags

subnet mask

,

networking

Topics

Miscellaneous Networking

,

Internet Protocols

,

Windows XP Operating System

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Answers

 

by: QlemoPosted on 2009-08-12 at 14:13:43ID: 25083097

You can't do that without changing the subnet mask, and changing the mask implies using static IP.
The DHCP server should propagate the "correct" subnet mask. The Server and your network need to be logically in a single network. The most restrictive subnet mask would be
255.255.128.0. I don't think the guys will accept that, as it is not far away from 255.255.0.0.

Another way would be to set the image server gets another IP address of your network. Having several IP addresses for a single server can confuse software running on that server, but I think it is worth a try.

 

by: fissionman1Posted on 2009-08-12 at 14:28:42ID: 25083227

Unfortunately the network admins in my company are running out of IP addresses and are not willing to give out any more than one per computer. What do you mean the DHCP server should propagate the correct subnet mask? Is there a way to force it to do this, since it obviously isn't now? Also, what do you mean by a restricitive subnet mask? I thought the mask 255.255.0.0 was a wider less restrictive mask than other values for the third octet, which is why I'm able to see all of my computers, instead of just a portion of them.

 

by: dosdet2Posted on 2009-08-12 at 14:59:43ID: 25083514

Hi fissionman1,
With no control over your DHCP and banned use of static IP addresses makes it a real problem.  Normally DHCP issues the mask information along with the IP address, DNS & Default gateway addresses.  I would guess that you do not have control over server and other device's IP information either.  

The only thing I would suggest (if you can physically plug into a switch port on each network) is to install a router between the 2 networks and then setup routing between the 2 networks from there.  This would probably be the lease intrusive and you wouldn't have to tell your admins that they are wrong.  (They don't like that.)

Contrary to what your network admins tell you, Those 2 IP address/mask combinations are NOT in the same subnet.  They need to go back and consult a subnetting manual or download Advanced IP Address Calculator at www.radmin.com and check it out.

The tightest mask that would put these even close to being in the "same network" is 255.255.128.0 and that is very iffy because of the 255.255.252.0 mask on the server.  (All devices on the same subnet should have the same mask.)  You may be able to communicate if you could get this mask on your workstations without using static IP's (which you really can't)  but If you could, I doubt it would be stable because of the different masks.  You would also have a lot of other address available to your network which could cause a security nightmare.

I hope this helped a little.

 

by: dosdet2Posted on 2009-08-12 at 15:06:07ID: 25083562

After reading the two comments above (When I started typing there were no comments yet), I wonder why they are using public addresses in their private network?  
No wonder they are running out of IP addresses.  Best they should start converting some subnets to the 10.0.0.0 series of addresses.  They could eliminate a bunch of problems.

 

by: QlemoPosted on 2009-08-12 at 15:08:20ID: 25083587

fissionman1,
You are correct. "Most restrictive" is meant as "as narrow as possible to allow for all needed IP addresses". That netmask would allow for the 202 subnet and 17x (and some more, in fact 128-255). The 255.255.0.0. would allow additionally for the lower half, 0-127.

DHCP server allow for an optional setting of the subnet mask. Else it will be set according to the network address class (255.0.0.0, 255.255.0.0, 255.255.255.0).

I agree with dosdet2, having nothing with which you deal makes the job useless. And about this admins - I'm ashamed of those, because I'm one too, and they are soooo wrong.
With the routing solution you will still have the problem that the "clones" and the server would be in different networks, and now even separated by a router, so no broadcast traffic will pass. I do not know how the Imaging software determines the clones, but I suppose it is a broadcast thing.



 

 

by: QlemoPosted on 2009-08-12 at 15:08:53ID: 25083593

:-) Again, concur!

 

by: fissionman1Posted on 2009-08-12 at 15:41:04ID: 25083821

Qlemo, dosdet2, thank you for your responses. You both seemed to have confirmed my suspicion of lousy network administration. So DHCP servers do have the option of specifying subnet masks on a computer to computer basis? That seems to be my only option as of right now.

 

by: QlemoPosted on 2009-08-12 at 15:49:04ID: 25083888

No, not exactly. DHCP server allow for general setting the subnet mask. You can have several IP pools with own settings and options, and setup computers to use one of them (by using so-called "class IDs"). You cannot set it for each computer. The only thing you can do on a computer base is to reserve its IP address.

 

by: fissionman1Posted on 2009-08-12 at 15:56:12ID: 25083934

Unfortunately my company also frowns upon using a router for anything but a firewall. So setting up a router is also out of the question. Would changing the 128.104.17X.XXX computers IP's to a addresses closer to 128.104.202.XX (so that they are in the same IP pool) fix the problem?

 

by: dosdet2Posted on 2009-08-12 at 16:03:20ID: 25083976

Yes, If you could change your server and "other devices" to be in the range of your clones, that would do it.  You would need them to be  between 128.104.202.001  and 128.104.202.254 with a mask of 255.255.255.0  
If you stay with IP addresses above or below the range of your workstations, you should be fine.

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