DHCP explanation 1

[serhino320]

Hi All!

I'm running DHCP on a test server where I work and I cannot figure out some issues with how ip numbers are issued and validated. Setup of DHCP went smoothly and the server is issuing ip's to the the client computers that are requesting them.


How do I restrict the issuing of ip numbers to soley the computers in my network. Right now if I plug a client computer into the network configured for DHCP, the server gives it one. I can then see it's MAC address in the server's list of address leases, but how do I restrict which computers are allowed to get an IP. I don't want just any person with a laptop plugging into my network and grabbing an IP. Is there some location that I input the list of MAC addresses that are allowed to lease an ip number? If I want a range of IP numbers made available to anyone I guess I can just set up another scope without MAC address validation, right?

Any help would be appreciated. I currently use static IP numbers on my network of 50 PC's (WIN XP)and am interested in going DHCP for RIS use.

Thanks

 

[GrnEyedLdy]

Serhino320,

I found this on DHCP-handbook.com...

"How can I control which clients get leases from my server?

There is no ideal answer: you have to give something up or do some extra work".

"You can put all your clients on a subnet of your own along with your own DHCP server.
You can use manual allocation.
Perhaps you can find DHCP server software that allows you to list which MAC addresses the server will accept. DHCP servers that support roaming machines may be adapted to such use.
You can use the user class option assuming your clients and server support it: it will require you to configure each of your clients with a user class name. You still depend upon the other clients to respect your wishes".

"How can I prevent unauthorized laptops from using a network that uses DHCP for dynamic addressing"?

"This would have to be done using a mechanism other than DHCP. DHCP does not prevent other clients from using the addresses it is set to hand out nor can it distinguish between a computer's permanent MAC address and one set by the computer's user. DHCP can impose no restrictions on what IP address can use a particular port nor control the IP address used by any client".


Patty

 

[serhino320]

Thanks Patty!

I guess DHCP is not as trivial as one is led to believe. My network environment is a university. In my building we have 3 departments that each have W2K servers that run DHCP and service client machines in their respective department. The ip addresses allocated to me by our campus network guys are in the XXX.XXX.74.XXX subnet. Dept2 has ip numbers in the XXX.XXX.64.XXX subnet and dept3 has ip numbers in the XXX.XXX.44.XXX subnet. Whenever dept1 or dept 3 clients machines request an ip number from a DHCP server we always get a number from dept2's subnet. We guess that it has something to do with how the router handles all of our connections. This is bizarre that I cannot stipulate that my client machines get there IP numbers from my DHCP server.

Shawn

 

[globalstrata]

All you need to do is reservations for every computer that you want to give an IP to. In order to do the reservation, you will need every computer MAC.

Click the Server\Scope, then right Click the Reservation and the add one for each machine.

Then all you have to do is add an exclusion Range under
Server\Scope\Address Pool. This Exclusion Range will have all the IPs that you do not want to give out.
Gladys Rodriguez
GlobalStrata Solutions

http://www.globalstrata.com

 

[serhino320]

That's It Globalstrata!

I was wondering what the work around was. Reservations and exclusion ranges will do just what I want. Star to you globalstrata

Shawn

 

[GrnEyedLdy]

Hi Globalstrata,

Just cruious, but...

Why use DHCP at all if you are going to give all clients a reservation...Why not just use static IP addresses?

Are there benifits one way or another?

Thanks,

Patty

 

[GlenJohnson]

Globalstrata was on the money. Let me guess, your setting up a home network to study for ms. That's what I've done. One dc, one 98 machine, one w2k machine, one w2k laptop that has a static ip so it can connect to the internet via a wireless router. Personally, I prefer dhcp instead of fixed ip's. My cable modem provider has lousy software, and when I start my machine, sometimes it doesn't find a nic. Don't know why, but I wrote a batch file for when this happens. Ipconfig /release Ipconfig /renew. If I can't get on the internet, I just run the batch file and it gives me an ip via dhcp. Works every time. Good luck. Glen A. Johnson
Johnson Computer Consulting
MCP W2K
glen@johnsoncomputers.us



"Uniformity is death, diversity is life."
Mikhail A. Bakunin (1814-1876); Russian writer and revolutionary.

 

[serhino320]

GrnEyedLdy,

RIS needs DHCP so I need to have DHCP running. I guess it does appear to be back to a static ip scenario but if RIS wants DHCP I can play the game.

BTW-Anyone have any suggestions regarding the multiple DHCP servers issue? What establishes one DHCP server as the primary ip issuer over another one. Does anyone else have this sort of scenario? Again- 3 departments each wanting their own DHCP server, but somehow each sharing the same pipe through the routers? Thanks

Shawn

 

[GrnEyedLdy]

If these 3 departments are on different subnets and each subnet is running it's own DHCP server, then I would think that each client would get a response from the server on their own subnet.


"The ip addresses allocated to me by our campus network guys are in the XXX.XXX.74.XXX subnet. Dept2 has ip numbers in the XXX.XXX.64.XXX subnet and dept3 has ip numbers in the XXX.XXX.44.XXX subnet".


What subnet mask are you using with these IP addresses?

Patty

 

[dankelt]

Shawn, in your case, I would use a relay agent to provide dhcp info to other subnets. I would use multiple DHCP servers only if you needed fault tolerance. I'll post some links with info on this:

http://support.microsoft.com/default.aspx?scid=KB;en-us;q120932

http://www.microsoft.com/windows200...elp/sag_DHCP_und_RelayAgentConfigurations.htm

Hope This helps!

 

[serhino320]

Patty-

Subnet mask is 255.255.255.0

Dankelt-

I'll check out the links. It seems to me that we must have a relay agent in place if a DHCP server on one subnet is servicing ip requests to clients on another subnet.

Thanks again everyone!