DHCP/Subnet help needed urgently

Ok so here's my problem.

I run a Windows 2000 server based domain using Active Directory.

My current network is setup using a single scope:

10.0.10.50 -> 10.0.10.249
Subnet: 255.255.255.0 (24)

However we are running out of available IP addresses in the scope (1% available).

I know the simple thing to do would be to change the range 10.0.10.1 -> .254 giving myself some more addresses, but this won't do in the long term.

How can I expand the number of IP addresses available to my local network?

Please go easy I genuinely am not networking trained, but I have no choice but to do this myself.

Thank you kindly for any help.

 

You can ignore this now, I bit the bullet, deleted my existing scope and recreated a new one with a 255.255.254 subnet and so far all seems well.

 

A couple of points to make here:

First, I'd recommend you bite the whole bullet and use a whole octet for the new subnet rather than a small partial subnet. I guarantee using 255.255.254.0 will cause you confusion in the future. Use 255.255.0.0 instead. It is a lot simpler. KISS is such a good philosophy in networking.

You can then use the third octet to identify PC using DHCP. So create DHCP scope options that give out 10.0.10.1 to 10.0.11.255. Then use 10.0.0.1 to 10.0.0.254 for servers and the like.

Second, there is more to expanding a subnet than just altering your DHCP scope. For example, consider this example. Your DHCP system gives out its last 10.0.10.x address (10.0.10.254). It then needs to give out another IP. Using a 255.255.254.0 mask, the next available address is 10.0.11.1.

So when a new PC comes onto the network it gets that address (10.0.11.1). If it then needs to send a packet to the internet, and it sends the packet to its default router - as given out by DHCP. Now this router is set up for the 10.0.10.0/255.255.255.0 subnet. As far as the router is concerned, the 10.0.11.1 PC is not a member of its own subnet, and it therefore replies to it via its own default route (probably on the external interface) rather than via it internal network interface. The result is that the new PC can't connect to the internet.

Same thing if the new PC tries to connect to a server that is statically set up to use 10.0.10.0/255.255.255.0. The server may receive the packet, but it will try to reply via its default route as 10.0.11.1 is not part of its subnet. so again, the new PC fails to communicate.

The key point is that you also have to update the masks on all statically assigned systems. That's servers, routers, printers and statically assigned PC. If you don't do that you are going to get all sorts of odd communication problems.

The key point:

10.0.10.1/255.255.255.0 will not directly connect to 10.0.11.1/255.255.255.0 and will need to connect to via a router that recognises both 10.0.10.0/255.255.255.0 and 10.0.11.0/255.255.255.0

10.0.10.1/255.255.254.0 and 10.0.11.1/255.255.254.0 are on the same subnet and will therefore communicate directly with each other (no router needed between them).

 

Thanks for the info ReggieB

As stated it isnt something I had done before, so used best guess.

For the moment I will stick with the .254 subnet as I don't want to change the static addresses of the servers as this is much bigger job and lots of things would need updating (lazy I know but hey ho )

 

Now, its done there's a good case for not changing it again if its hard work on the servers. 255.255.254.0 will work OK, but I think it is worth making the point that 255.255.0.0 is simpler and more flexible. Especially as they may be others wanting to follow your example.

Good news that everything is working ok after the change.

 

point deffinitely noted should really have gone that far, but was too eager.

Things still seem well apart from I forgot to reserve the address of our main file server which then meant somebodies machine took it's address and called all kinds of network havoc.

all is well and good again now.

think we really need to emply a network guy :/

 

It's worth creating a small range of addresses that are excluded from the DHCP scope - and use these for the devices you need statically assigned. You can either just make the scope cover a limited range of addresses (for example, most small routers limit the addresses they give out to the range from 100 to 200 - e.g. 192.168.1.100 to 192.168.1.200) or add an exclusion range. At a very minimum, I would exclude 1 to 20 for use by static nodes.

It is far easier to manage a system where there is a set range of addresses that are only static and a different range that are dynamically assigned. That's far easier to manage than reserving individual addresses.

Personally, I'd only use reservation to assign an address to a device that is difficult to assign manually - for example some cheap print servers.