The growth of the amount of wireless devices on our network has accelerated. Everyone–especially the students–has every reason to celebrate. But the IT Dept isn’t in the same celebratory mood. The reason? We’re running on empty (or, more precisely: the DHCP server is).

The problem is that the DHCP server is fast running out of IP addresses to dish out to all the new devices being added to serve our students and faculty. There’s an exclamation mark hanging like an ill omen over the DHCP server icon, an indication that we’re dangerously close to the end of the available address pool.

Thankfully, there’s a simple solution–using a superscope. What’s a superscope? A kind of mother of all scopes. It allows you to add more than one scope (called child scopes, or member scopes) under one umbrella.

Let’s go ahead and create a superscope for our scenario. We’ll assume DHCP is set up to use the scope 192.168.0.0. We want to add another scope from the same class (Class C), so let’s use 192.168.1.0. But first we need to create a superscope. Here’s how:

  1. Open DHCP.
  2. Right-click on the DHCP server.
  3. From the drop-down list, choose New Superscope to launch the New Superscope Wizard.
  4. The wizard prompts you to enter a name for the superscope. We’ll just call it Wireless.
  5. On the next screen, you’ll be asked to select a scope(s) to add to the superscope. You’ll see the list of available scopes–in our example we’ll just use the original IP range of 192.168.0.0. Select it and click Next.

The final screen of the wizard informs you that you have successfully completed the New Superscope wizard and gives you the details. If you go back into DHCP, you’ll see that the new superscope has been created.

Now we’re ready to create our brand new child scope that will be watched over by our superscope.

  1. Open DHCP.
  2. Right-click on the DHCP server.
  3. Select New scope (to launch the New scope wizard.
  4. Choose a name and description for the new scope. Call it whatever you want.
  5. The wizard will prompt you to add an IP address range. We’ll choose a range from the Class C range 192.168.1.0. (We could also have chosen 192.168.2.0, 192.168.3.0, etc., but we’ll stick to … 1.0, as it follows logically on our first range). As for start and end address, we’ll select all available addresses, starting with 192.168.1.1 and ending with 192.168.1.254. You will also have to use the arrow keys to update your netmask (most likely to 255.255.255.0).
  6. On the next screen, you can choose which range of addresses you want to exclude, if any.
  7. Now, you get to select the duration of IP address leases. The default is eight days.
  8. The wizard then gives you the opportunity to configure DHCP options. You can choose to do it now or wait until later. Note, however, that you have to configure the most common options (like DNS server address and default gateway) before clients can use the scope, so now is as good a time as ever to do it. Just use the same options as your existing scope.
  9. After configuring the DHCP options, you are asked whether you want to activate the scope now or later. Once activated, you’re almost done.

You now have what is termed a multinet–multiple subnets on a single physical network. But you’re not quite there yet. Yes, you have an additional scope; yes, you have a superscope. But your superscope won’t assign IP addresses from the new scope. And even if you add a static address from the pool to a client machine, you’ll notice that you can’t browse the network.

You still need to add the IP to your main router. Here’s how to add that:

The commands to add an IP address to an interface look something like this (depending on the interface and address):

int e 0/0

ip address 192.168.1.0 255.255.255.0

But you’re adding a second address to the same interface, so you have to add the keywordsecondary to the command. So to add the address range from our new child scope, the command would be:

int e 0/0

ip address 192.168.1.0 255.255.255.0 secondary

That’s it!   When your first series of IPs run out the devices should grab a hold of the second range.