DHCP on Primary IPs subnet only?

[Anil Gupte]

This is a little hard to describe, so please bear with me. My Windows 2000
server (which incidentally is my Active Directory DC ), will provide DHCP
address in the w.x.y.0/24 range, but not in the a.b.c.0/24 range. Its
primary IP is w.x.y.z However, it will not provide DHCP in the a.b.c.0/24
range. I put in a secondary IP (in TCP/IP properties - Advanced) in the
network properties, of a.b.c.d but it will still not hand out IPs in that
range. I did disable the old scope.

Seems like it is trying to tell me that the Primary IP on the machine has to
be in the subnet that I am assigning DHCP in. Is this true? If so, it
sucks. I realize that DHCP being a broadcast protocol, it needs to have
"an" IP on the same subnet as the DHCP clients, but why does it have to be a
"primary" IP. Is there any way around this or a fix to this?

Thanx,

 

[Marc Reynolds [MSFT]]

Is the DHCP server on the same physical segment as the a.b.c.0 clients? If
not you need to set up the router that divides the segments to do DHCP relay
(or bootp relay).

--

Thanks,
Marc Reynolds
Microsoft Technical Support

This posting is provided "AS IS" with no warranties, and confers no rights.

 

[Anil Gupte]

Yes, the DHCP server is on the same physical segment as the a.b.c.0 clients?
Same clients and server work fine if I am using the w.x.y.0 scope. and the
server IP address is w.x.y.z - which made me think that the scope has to be
on the same IP subnet as the Primary address of the DHCP server. Is this
true?

Thanx,

 

[Lindsey]

You should use a switch/ router to seperate your vlan's /
scopes. On the switch use the IP helper command to point
to your DHCP server. Although the workstations are
broadcasting a IP request, the swtiches should never pass
this broadcast on to the dhcp server. It should only pass
the request with the end users MAC address. An IP will
then be assigned to this MAC address and passed back to
the end user. If you need more specifics please post. Good
Luck.

 

[Anil Gupte]

Sorry, but I don't think that is the issue at all. We need only one scope.
However, it appears that Windows will not hand out IPs in a subnet unless
its primary IP is on that subnet. That is or was my problem. For now, I
have renumbered all my Servers with a new IP, but it is a pain and a kludge.

Thanx,

 

[Matt Hickman]

Sorry, but I don't think that is the issue at all. We need only one scope.
However, it appears that Windows will not hand out IPs in a subnet unless
its primary IP is on that subnet.

So the DHCP server sees a request for an IP address. It knows it
comes from the subnet it is on because the broadcast does not go
through a forwarder that attaches subnet information to the packet.
It knows its own address and subnet mask, so it knows the valid
addresses it can assign. But you have disabled that scope (w.x.y.0/24)
so it doesn't reply. Sounds like it is doing what it should.

Try assigning the DHCP server's NIC a second static IP address from
the a.b.c.0/24 range. It should see the requests comming in from
that IP address and be able to assign addresses from the a.b.c.0/24
scope that you set up and activated.

 

[Matt Hickman]

Sorry, but I don't think that is the issue at all. We need only one scope.
However, it appears that Windows will not hand out IPs in a subnet unless
its primary IP is on that subnet.

So the DHCP server sees a request for an IP address. It knows it
comes from the subnet it is on because the broadcast does not go
through a forwarder that attaches subnet information to the packet.
It knows its own address and subnet mask, so it knows the valid
addresses it can assign. But you have disabled that scope (w.x.y.0/24)
so it doesn't reply. Sounds like it is doing what it should.

Try assigning the DHCP server's NIC a second static IP address from
the a.b.c.0/24 range. It should see the requests comming in from
that IP address and be able to assign addresses from the a.b.c.0/24
scope that you set up and activated.

 

[Anil Gupte]

Quote: "Try assigning the DHCP server's NIC a second static IP address from

the a.b.c.0/24 range. It should see the requests comming in from
that IP address and be able to assign addresses from the a.b.c.0/24
scope that you set up and activated."

I did exactly that, and it does not work. I even tried to delete the
w.x.y.0/24 scope so it would not be confused. Finally, I changed the
Primary IP of the machine to a.b.c.d and it started handing out addresses in
that range (a.b.c.0/24).

 

[Matt Hickman]

Quote: "Try assigning the DHCP server's NIC a second static IP address from

I did exactly that, and it does not work. I even tried to delete the
w.x.y.0/24 scope so it would not be confused. Finally, I changed the
Primary IP of the machine to a.b.c.d and it started handing out addresses in
that range (a.b.c.0/24).

Do you still need the DHCP server to answer to the w.x.y.z address?
Can you make that secondary and the DHCP still assign adresses from the
a.b.c.0/24 range?

Not sure why MS would build in the 'feature' of using only the primary
IP address (or only) to determine what scope to use.

 

[Anil Gupte]

Yes, that works. I made the addresses in the w.x.y.0/24 range to secondary
addresses and as described made the a.b.c.d address primary, and it started
to hand out DHCP addresses in the w.x.y.0/24 range. I guess it is a
"feature". Perhaps the server listens for broadcasts (that is what DHCP
requests are) only on it's primary IP. Otherwise it might get flooded with
broadcasts.

It is a moot point now since I have made permanent changes, but I would be
curious to learn what the rationale and correct behavior is.

Thanx for your advice.