Expanding network to another subnet

Posted on 2014-03-21
Last Modified: 2014-05-13
     I manage a small business network and we are running out of IPs. I am wondering what the best practice would be for expanding into another subnet? I currently have a sonicwall NSA220 router and a Windows Server 2003 DHCP server. What I would like to do is have another subnet for new workstations and servers, and have these subnets talk to each other. What would be the best way to go about this?

Question by:indigo6
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LVL 30

Expert Comment

ID: 39946792

Expert Comment

ID: 39946824
I would either expand the subnet which you will have to setup a new  DHCP scope on windows 2003 or create the other subnet and run DHCP on the Sonicwall or if the Sonicwall supports IP  helper addresses configure an addition DHCP scope for the new subnet on Windows 2003.
LVL 37

Expert Comment

ID: 39946895
> So you are using up all 253 class C  IP's?

hehe, that doesn't sound like a small business?

how are the client computers connected to the network? wireless or wired to other switches then to the router?

generally, i think there are two approaches depending on your cost, time and security considerations.

the first is to simply add one more subnet, define the router to relay DHCP to the new subnet or even enable the router's DHCP services against the new subnet.

another approach is to refine the whole network per your organisation structure and server infrastructure, this may allow the network to support your business for an even longer time.

for example, move all servers to one or more subnets hence more protection could be applied based on security zones (subnets). the same principle may apply to the client computers too depending on different security levels for different departments or groups. a central router is used to interconnect all subnets with proper firewall rules applied per security and performance considerations.

of course the latter approach may cost you more on money and time, but may streamline the network to meet your business needs in the near future.
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LVL 69

Expert Comment

ID: 39947349
The most simple approach is to just expand your subnet mask by one or more bits. Instead of /24 ( use /23 ( or /22 ( Each bit doubles the available amount of IPs.
You can set that up directly in the DHCP scope and options.

However, too many devices in the same subnet cause a lot of "noise", reducing the available bandwidth. That does not happen if using separate subnets connected by routers.

Author Comment

ID: 39947940
Ok, thanks! I think I want to go with a second subnet. What I would like to do in theory is have another DHCP pool on a different server for all the new dynamically assigned addresses, and then have my existing subnet be just for my servers, etc. Can I do this with just one router? How would I configure my sonicwall if so?
LVL 37

Expert Comment

ID: 39948010
how many servers do you have?

is a single class C subnet big enough for all your client computers?

how many zones does your SonicWALL support for routing?

Author Comment

ID: 39948077
We have a quite a few servers, but we are a print shop so we also have quite a few statically assigned devices like platemakers and digital presses. A single class C will be sufficient.

I'm not sure how many zones, it is a NSA220
LVL 27

Expert Comment

ID: 39948569
While I do concur with expanding changing your network from a standard class C to having a 23 bit subnet mask, there are two approaches you can take to create a second subnet:
1) Configure one of the LAN ports with a new subnet
2) Create a VLAN

The first one tends to be simpler as long as it is easy for you to do a physical separation of the network equipment, otherwise the VLAN approach is the only feasible solution.

To do choice #1, you configure one of the ports to be part of a new separate subnet (say port X2 has an IP address on subnet, then you can create a new DHCP server if necessary.

Choice #2 involves creating a virtual interface, as well as configuring your switches to understand the VLANs.
LVL 16

Expert Comment

ID: 39950580
Another subnet means that you have to configure your router(s) for this second subnet, and that the "new" devices will have to connect to the "new" subnet one way or another.
You will also need to configure ip-helper addresses (DHCP relay) on your routing devices so that the DHCP discover packets sent by DHCP clients to broadcast addresses on the new subnet are forwarded to your DHCP server (broadcasts are not routed and broadcasts sent on one subnet do not reach hosts in another subnet).

Expanding the subnet mask by 1 or 2 bits is certainly an easier path if networking/routing/subnetting/vlans is not something you want to play with.

Check this article for some more details and operations :

Author Comment

ID: 39951260
Ok, I certainly agree that expanding the subnet would be easier. However I have more than 100 manually configured devices that I would have to change the subnet mask on. So I'm note sure which solution would actually be easier. Also, I'm not sure if all my switches support VLANs.
LVL 16

Accepted Solution

vivigatt earned 500 total points
ID: 39951611
Depending on your routing devices, you MAY keep the statically configured devices with their current config (at least temporarily).
If this works, this is how I see things:
when one of your "static devices" (Node A) sends a packet to another node (node B) in your LAN, if node B is in the "subnet range" of the current subnet mask (between and for instance), everything stays the same, Node A resolves node B's address using ARP and sends packets directly to it.
Now if Node B is in the "new range" (let's say  its address is, node A is still using the old subnet mask ( and then "thinks" that node B's is in another subnet. It then sends the packet to its default router. The router receives the packet and... forward it to Node B. This is not very clean, this adds an hop to the path the packets take, this put some pressure on the router but this may leave you some time and flexibility to change the "static nodes" subnet mask on an ad-hoc basis or without having to do them all in a row.
When you change the static nodes config, maybe you can configure them as DHCP clienst and use DHCP reservations to make sure they always get the same IP address.

Now, that being said, if you just can extend your scope for it to have more available addresses, this would be ideal. No subnet change, you just extend your scope. Let's say that you currently have, in the network, the following scope : If you can use the scope, you then have more than doubled your pool of available addresses.

Author Closing Comment

ID: 40062279
This solution was the most complete, thank you!

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