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Community Member

dhcp and different subnet design

We are trying to determine what is the best design for subnetting. We are moving into a building that has 4 floors. We are considering whether we should create class C subnet for each of the floors or just rather create a supernet of 4 continuous class C subnets. We are also using dhcp to assing IP to the user computers.

I am wondering if a user with his laptop disconnects from the network without releasing the IP and not turning the computer off and then goes to another floor and connects to the network, if the computer will automatically pick up a new IP in the appropriate subnet? We have windows 2000 and XP OS. I have heard some saying that it is automatic and others saying that you have to release and renew. I would appreciate if I can get a straight story from someone here. Thanks for the help.


Re: dhcp and different subnet design

Question is how many users you have in total on all floors, plus what kind of applications you use, that justify the use of 4 vlans. You can use a separate vlan per floor to restrict traffic within each floor to its own broadcast domain.

Now for DHCP you can either use an external DHCP server or use Cisco IOS dhcp server on a Cisco router or your layer 3 switch.

A PC when disconnected from the network, the IP that it got via DHCP will be removed from its nic immediately. So if you do an ipconfig, you wont see that IP. But in the registry of the PC it caches the previously obtained IP and hence, it will ask the DHCP server for the same IP. You wont have to do a release and renew. DHCP will automatically get the IP from the appropriate subnet.

Community Member

Re: dhcp and different subnet design

For your question, there might be at most 90 users per floor. That isn't much but require at least two class C subnets.

It is good news that the user computer will automatically get an IP from the appropriate subnet without user intervention. I have heard from some that this does not happen. That means that the DHCP server and/or network are not properly configured based on your answer.

We are using Active Directory with dynamic dns (DDNS). Is it possible for Cisco routers and switches to also perform DDNS? We need to register the names of the computers into our DNS when an IP is assigned. Thanks for your help on this issue.

Re: dhcp and different subnet design

DDNS is not something that Cisco routers handle, it will be taken care of between the PC and the Domain Controller (or DNS). I have seen often that DDNS does not properly refresh the information when IP address changes. May be a log out and log into the domain on the client PC while connected to the new subnet, might help in updating the DDNS info.

Community Member

Re: dhcp and different subnet design

Thanks for your help.

Re: dhcp and different subnet design

How many switches are you looking at and how will they be connected? I think this will determine what the VLAN/IP Design will be.

Generally each logical access switch should have a unique user VLAN (and a Voice VLAN if you are deploying voice as well?). So for example if you have a 2950 with 48 ports then you should have an IP network with a /26 subnet mask (max 62-hosts). If you have a stack of two 3750's with 48-ports in each you should have a /25 subnet (max 128-hosts).

The issue you mention about DHCP should not be a problem; if the User moves his PC into a different network the DHCP request should be NAC'd by the DHCP server and he should be offered a new IP address in the correct subnet. Windows 2000 & XP detect the network connection so if you disconnect the cable it shuts the interface down. When you re-attach the cable Windows will attempt to keep the DHCP-assigned address by contacting the DHCP server, if you are on a different network the DHCP server will NAC the request and offer a new IP address. This used to be a problem with NT 4 and WIndows 95 as they didn't detect the network connection.



Community Member

Re: dhcp and different subnet design

It is interesting you mentioned 48 port 3750 stacked. That is what we will be using. However, in the near future, we will be adding more ports since not all wires will be active from the start. For that reason and for simplicity, we are using class C subnets.

Thank you for confirming the dhcp issue.

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