Posted on 2004-08-05
Last Modified: 2010-04-11
I am moving networks in my organization from a class b network to a class c, well sort of.

I have a subnet of 172.23.224.x I need to create a dhcp scope on the higher end of the network. That is 172.23.226 and 227. The question is, do I need a superscope or just a regular scope?

I created one as follows.

range 172 23.227-250
exlcusions and 255

When I activated the scope and try to renew the IPs from clients. I wasn't able to receive one.  However, I notice that on the dhcp server the leases were appearing. So it looks like the server was receving the ack but the clietn wasn't.

I looked at routing and ip helper address and they seem to be ok.

Any ideas?
Question by:jibarra
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Expert Comment

ID: 11731651


Usually just an ip/ release and renew would do it.
You could push it to a soft reboot if you can.
I doubt you need a superscope.
If the release doesn't work, try ipconfig/ flushdns
ipconfig /registerdns

Author Comment

ID: 11732773
thanks, but that is not the issue. I now suspect is the way my subnet is setup.
The network is 172.23.224.x/22
wich gives me 172.23.224-227
i have reserved 172.23.224 and 225 for static ips that works ok. My dhcp range is 172.23.226*27.  with a gateway of

Reboot nor release work, however, as i stated before. I see the client name registering on the dhcp leases.

any other ideas?

Expert Comment

ID: 11733018
Hi Juan,

Let's see if we got this.

You have a network that you want to have some portion dhcp serviced (172.23.226-227.*)

Is that correct?

Are you planning on keeping this a single network, or are the 226-227 addresses going to be a new subnet?

If there is no new subnet, then the DHCP devices on the network will just receive 226-227 addresses.  That means to say that the mask is not /27, but still /22.

If you are planning a new subnet, then we need to know what the IP address of the DHCP server, as well as its mask and default gateway (router).  If the dhcp server is on the new /27 net with your clients, then it should all work just fine (but it sounds like it isn't working just fine ;-).  If it is across a router (on a different network), then

   the router will need to be configued with a interface in the /27network and the network of the dhcp server
   the router will need a helper to point the dhcp requests to the server
   the dhcp server will need to know a route back to the new /27 network.

Let's start with that and see where we go


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Author Comment

ID: 11736815
Troy, i will not be creating a new network, I need to use the 22 subnet. Which the routers already know about as 172.23.224..0/22.  I also have the helper address on the router pointing to the dhcp server.

I have two other new networks with /24 subnets. and they are working great.  No problems at all via DHCP. so I suspect is either the way the /22 mask is broken down or router issue.  


Expert Comment

ID: 11736869
Ok, so we are keeping all of the devices in the /22 network, including the DHCP-serviced devices.  
What is the IP address/mask of the DHCP server?
What is the route table for the DHCP server?
Can you ping from the DHCP server to the static clients on the /22 net?

 (I presume that the static and the dynamic devices on the 22 net actually share the same physical network; i.e. you could give one of the dynamic stations a static address in the /22 and they would work OK?)


Author Comment

ID: 11737277
The IP address of the DHCP server is still on the old network. However, I have a172.23.30.0/24 (new network) DHCP configured on it and it works ok.

I am able to ping from the server to the static clients on the /22 subnet.


Accepted Solution

tropsmr2 earned 500 total points
ID: 11737697
Hi Juan,

Well, the soup gets thicker:-)

Let's summarize:

Your dhcp-served /22 clients are requesting an IP address.
since the DHCP server is not on the /22 net, the intervening router has a helper configured
The DHCP server indicates that it has received and issued a lease for the /22 client
The client does not get the lease.

DHCP is a 4 packet process:
DHCP Discover (client)
DHCP Offer (server)
DHCP Request (client)
DHCp ACK (server)

If I remember correctly, the Offer is (IP) unicast back to the helper (router), the ACK is unicast back to the client.

Your problem description seems to suggest that there is a problem with either the Offer or ACK reaching the client, thus a return path problem from the server to the /22.

Your ability to ping from the server to the static /22 clients seems to indicate there is no return path problem.

We could probably continue troubleshooting this problem for some time.  Are there other routers inline, what is the configuration of the DHCP server for the /22 net, on and on .

The best and fastest way to resolve the issue is probaly to get a sniffer trace of the problem.  

Can you install Ethereal ( on a PC and place it on a HUB with the DHCP server.  The output from that trace will tell us if we have a server config problem or a network problem...

Let me know if you have any questions about that.  A sniffer is an invaluable tool for anyone running a network.  Cannot live without it!



Expert Comment

ID: 11737756
one other thing occurs to me....

Just because you can ping from the DHCP server to the static addresses does not necessarily  mean that it still isn't a return-route problem as the DHCP address range (absolute)  is different than the static.  Before you sniff (if that's a big effort), take a PC on the /22 and statically assign an address in the DHCP dynamic range.  Then ping *that* from the DHCP server...


Author Comment

ID: 11755707
Ok, Issue resolved.
I ended up recreating the scope on a server already in teh /22 subnet, and voila,  everything worked.
So i just left it there as my timing is crucial right now
thanks to all who helped here, your comments were on the right track!!!!


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