Cisco Switch -- multiple networks on one switch ?

My current Windows DHCP server is running out of IPs,
therefore I was planning to change the SubnetMask from
255.255.255.0 to 255.255.252.0 on 100+ static servers,
routers, VPN tunnels, etc but do NOT have time to
right now, therefore I was just going to assign
a few statics in a new 25.x range.

I assume the below DESIRED is doable
without changing anything on my network ?

If not, what is doable ?

=================================================

CURRENT
  ** one DHCP server, 192.168.24.x
  ** one gateway, 192.168.24.x
  ** one subnetMask, 255.255.255.0
  ** one Cisco switch, 192.168.24.x
  ** multiple static clients, 192.168.24.x
  ** multiple DHCP clients, 192.168.24.x

=================================================

DESIRED
  ** same as above, but multiple static clients
     also getting assigned 192.168.25.x IPs
finance_teacherAsked:
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Wayne88Commented:
Yes you can.  You can implement superscope: https://technet.microsoft.com/en-us/library/dd759168.aspx

In addition, also verify that if your Cisco switch supports "IP helper".

"Each subnet that does not have its own DHCP server will be configured with an ip helper-address command. The configuration for this command follows; note that you first must change the router interface to which you will be assigning the helper."

For example, if your DHCP server is 192.168.1.1 and the switch is the default gateway on Vlan10.

interface Vlan10
 ip address 192.168.2.254 255.255.255.0
 ip helper-address 192.168.1.1
end

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Fred MarshallPrincipalCommented:
A dumb switch will handle this fine.  So, I would expect smarter switches to probably do the same.  But maybe some would not.

A simple way to think about it:

A "LAN" is a Local Area Network as you know.  The best way to think about this is "copper wire".
The copper wire and dumb switches don't care at all what's running through them.

A "SUBNET" is an address range with some rules attached.
You can run multiple subnets on the same copper wires and switches at the same time.
In most cases there won't be any interaction between subnets on the same LAN.

A "VLAN" is supposed to be a Virtual LAN.  So that would mean "just like copper wires".  What this means *in the simplest sense* is that the switches with VLAN capabilities at a minimum isolate their ports so that different copper wires are NOT connected through the switch.  Consider this simple model:
A 20-port switch divided into 4 5-port VLANs should look like 4 5-port switches .. just in the same box.
Now it should be rather obvious that this sort of thing isn't entirely useful, except to get your thinking straight, so from an operational and functional point of view, a "VLAN" or set of them has other features that cause them to be more useful and to interact.

So, if you intend to stack subnets on top of one another on a single LAN and you're using VLAN capabilities then you should be suitably careful that you understand what's what.
Wayne88Commented:
Hello,

Given the scenario  finance_teacher provided where there is only one DHCP server serving multiple subnets my ideas are:

IDEA 1

1. Configure the DHCP server to support the additional subnet
2. Create a VLAN for each subnet
3. Apply IP HELPER for nodes on the new subnet to retrieve the appropriate IP address from the DHCP server.  IP HELPER is needed on the switch to allow the IP address retrieval for the DHCP clients in a different subnet than where the DHCP server reside

OR

If you do not prefer to change anything on the network and run everything on the same LAN/VLAN (I don't like this configuration because it's messy but you can keep everything as it is).

IDEA 2

1. Configure the DHCP server to support the additional subnet
2. Assign IP reservation for each node on the new subnet

Wayne
Fred MarshallPrincipalCommented:
I guess your idea was to stay with 192.168.24.0/24 and to add 192.168.25.0/24 and to handle the addition by changing the subnet mask - which, fortunately will work.  Then, in the interim to save time, you would assign static addresses in the 192.168.25.0/24 subnet.

You might think about how much effort is needed:

If you have 100+ devices with static addresses and are concerned about having to change each one individually to at least a new subnet mask, you might consider:

- Instead of changing the subnet mask, change the method of addressing to DHCP.  That's fairly quick and easy and can be done with a script.
- In addition, capture and add the MAC addresses of these devices so that they can get the same addresses as "reserved" addresses from the DHCP server via DHCP.  That way they stay static and you get the advantage of dealing with DNS across the network.

Having done this then a large part of the work is done once and for all.
And, of course, this would be using 192.168.24.0/23 - so you have a bigger subnet range.

Clearly I'm not saying anything that others haven't said here but just in a different way  perhaps.

Something else to consider:
If you change the subnet mask to 255.255.254.0 on the DHCP server and the internet gateway right away, you can likely transition the subnet mask or DHCP settings on other devices in a rather leisurely fashion.  That's because the IP addresses will continue to work.  The network address will stay the same.  The device IP addresses will all work.  Devices still in 192.168.24.0/24 will send  packets destined for 192.168.25.0/23 to the gateway which will put them back on the wire.  Devices in 192.168.25.0 and above with subnet mask /24 won't exist.  Devices in 192.168.24.0/23 that are at 192.168.25.0 and above sending packets to 192.168.24.xxx will simply put them out on the wire and they will arrive just fine.  Devices that evolve into 192.168.24.0/23 that have addresses in 192.168.24.xxx will do the same.

However, the broadcast address will change.  So, one might avoid using 192.168.24.255 for a device in the interim.  Things needing broadcast will work up to the point that current devices with 192.168.24.0/24 won't recognize some new broadcasts (such as from the router and from devices in 192.168.25.xxx) but will recognize broadcasts from their peers.  The point is that you can judiciously select the order of devices to have their settings changed and likely not even notice that they don't match for a time.

So, I'd stay away from 2 VLANs and just go for the larger subnet once and for all.  The risk and pain is less it seems to me.
Wayne88Commented:
Actually no, you wouldn't need to set a static address for the nodes in the new subnet addition.  This is why I recommended using the IP HELPER for the VLAN switch serving the new subnet so it can be served by the same DHCP server in the original subnet.  There will be no need for statically assigning IP address in the new subnet.

However, I agree that expanding the subnet is probably easier and keep everything else the same.
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