DHCP, Scopes and Subnets

[SCLDTony]

Ok, first time setting up a Win 2K DHCP server. I have it working correctly on the local subnet. I have set the ip_helper setting on all the Cisco routers. BUT how does a request from a subnet other than the local "know" to request an address from the correct scope.

For example, how does a machine in the 255.255.255.193 subnet know to take an address from the C ("c" is the descriptive name of the scope, just for example) scope that has the proper router and subnet information (scope options) and not the A or B or any other scope?

Did I make any sense? Let me know if you need more clarification (Im sure you will!) Thanks!

 

[GrnEyedLdy]

SCLDTony,

From Mark Minasi -- Improving on DHCP

"Another consideration: If a DHCP server serves several subnets and its adjacent routers support bootp forwarding, the server must expect to receive DHCP discover broadcasts from any one of those subnets. So how does the DHCP server know which subnet the broadcast came from--how does the server know which subnet range to draw from when assigning an IP address to a client?

The answer lies in how bootp forwarding works. A bootp forwarding-enabled router will retransmit (forward) a DHCP discover broadcast. But when this router forwards the broadcast, it adds data, a note saying, "To anyone who hears this: This is a broadcast that I originally found on a different subnet, subnet x.y.z.a." Then, if a DHCP server receives a broadcast that was retransmitted over one or more routers, the server will know what subnet to direct the response back to and which scope to pull a number from for its offer."

Here is the link to the rest of the article,

http://www.winnetmag.com/Articles/Index.cfm?ArticleID=2662&pg=2

Hope this helps,

Patty

 

[SCLDTony]

Thanks for the information ) BUT... in theory, it looks like it should work, but in practice I am pulling my hair out. Here is what happened, and any more assistance would be appreciated:

We have a building that has a subnet of 255.255.255.192. I tried creating a scope for this building of address ranging from 10.40.10.100 - 10.40.10.200 When creating the scope in Win 2K, I got an error stating that I cannot use that range of addresses with the 255.255.255.192 subnet. Three superscopes were created. The building was not able to obtain a DCHP address. What am I doing wrong? Thanks in advance for assistance!

 

[GrnEyedLdy]

SCLDTony,

If you are using 255.255.255.192 for a subnet mask, with the IP address (10.40.10.x/26), then your subnets will be

Network 10.40.10.0
Host Address Range 10.40.10.1 thru 10.40.10.62 (10.40.10.63 = Broadcast address)

Network 10.40.10.64 Host Address Range 10.40.10.65 thru 10.40.10.126 (10.40.10.127 = Broadcast address).

Network 10.40.10.128 Host Address Range 10.40.10.129 thru 10.40.10.190 (10.40.10.191 = Broadcast Address)

Network 10.40.10.192 Host Address Range 10.40.10.193 thru 10.40.10.254 (10.40.10.255 = Broadcast Address)

By configuring a scope from 10.40.10.100 thru 10.40.10.200, you have overlapped networks and therefor your scope was broken up into three networks inside a SuperScope.

Does this make sense?

Patty