Running out of IP addresses

We have a single subnet of . with a subnet mask of

We are now running out addreses and want to increase the number of available addresses by changing the subnet mask to

What are the things i should consider before making the changes.

BTW, there some layer2 switches also in the network. Will these pose any problems after te changes?

There are no routers within the network

There is only one physical network spread over couple of offices interlinked by fiber
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Pete LongTechnical ConsultantCommented:
Layer 2 switches are unaffected by IP addresses they deal in MAC Addresses (If they were Layer 3 switches using VLANS then you would have a problem and need to chage their config)

If you are all on one site then You will be fine make sure there is no issue with your Router/Firewall (ie it can support CIDR (Pronounced Cider = Classless inter domain Routing)

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Guy Hengel [angelIII / a3]Billing EngineerCommented:
making the subnet mask greater is no problem at all, except that you need to adjust it on all the computers.
if you have DHCP and let the DHCP give the subnet mask, you simply change it there and on all the computers that have fixed ip addresses...
no problem go a head, but if you have static IP in some of your pc then you have to change the subnet

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lrmooreCommented: is a Class  C subnet with default mask of
Changing the mask to /23 is technically super-netting.
Not all IP stacks recognize supernets. Most versions of Windows is OK, most versions of Linux are OK, some printers may choke on it. Switches/hubs don't care at all. Some routers may choke on it if you try to add one in the future.
Since you have to touch every device anyway to change the mask (every single device must be the same mask), you migh consider changing to a Class B and subnet it. i.e. /23
Agree with Lrmoore, If you understand what is supernetting only then go ahead with it. Otherwise, create another subnet and just route them. You get an additional 254 hosts in there.

If it is too small then go with a Class B network.

Adding more and more hosts to the same subnet may cause some performance issues. You may just want to consider to break up your network in smaller pieces, ie in subnets. The obvious choice would be to define one C-class network as a backbone, which would connect a set of routers. Ech router would manage a C-class network in an office. This will also greatly enhance any trouble shooting when this ever may become neccessary...
masoodAuthor Commented:
How do i achieve that
Achieve what? Another subnet ? Just add it and make sure you have a router to route it.

masoodAuthor Commented:
Do i need to add a router even if it is in the same physical network?
Yes, to route between 2 networks you need a router. Now you would be already having a network setup right? So like if it is a Cisco Router, I know for sure that all you need is one port on the router to route to both networks.

Describe what you have right now.

masoodAuthor Commented:
I have created an additional scope of but doesn't seem to be working. Even my first scope gets full, clients are not picking up addresses from the new scope.

Am I missing smething somewhere?
Use as a netmask.
masoodAuthor Commented:
subnet mask is set to
Just for testiong purposes, I have assigned a stati IP address from the subnet 192..168.2. to a machine and it is working fine. Only issue is that clients do not receive IP addresses from this scope
> I have created an additional scope of but doesn't seem to be working
How have you created this new additonal scope?

Have you configured a DHCP server for the new subnet?
It may be helpful if you provide the complete network setup:
- what system provided DHCP services?
- in which subnet is the DHCP server located?
- what equipment do you use the separate the two subnets? Or are you using the same LAN with 2 subnets?
masoodAuthor Commented:
I am using the same LAN with 2 subnets has the DHCP server
using windowos 2003 DHCP server in windows 2003 domain
You may have to give your Windows 2003 DHCP server an additional IP address in the new subnet.
masoodAuthor Commented:
YOu mean I should have 2 NICs and assign 2 IPs from the two subnets?
You can add an additional IP address to your existing NIC:
Start -> Settings -> Network connections
right click on the network connection you are using -> properties
Select TCP/IP -> properties -> Advanced
In the top window you can add an extra IP address.
masoodAuthor Commented:
Didnt help. Situations still the same
What are the exact settings that you gave the DHCP server for the second 192.168.2 subnet?
More specifically what default gateway have you configured in the DHCP server for this scope?
- If you have configured a default gw for this scope on the 192.168.1 net (eg then these systems will not be able communicate with the rest of the network, since they cannot reach the router in the 192.168.1 subnet
- If you have confugured a default gw for this scope on the 192.168.2 net then you need also configure an additional IP address on your router, eg

Just for visualization: your current network looks like something this:

      |                  |                |                     |                     |         
    PC1             PC2            PC3                 PC4              DHCP server

The separation into mulltiple subnets this way, using the same physical lan just makes it complicated. My recommendation earlier for 2 separate subnets implied a physical separation of the subnets. That way you can better manage your network and have less broadcast traffic. If that is not possible then I would advice to just go with a larger subnet, eg define a subnet like /23, as recommended by lrmoore. If you do that then you will not have to dual home the DHCP server and the router.
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