New Subnet Creation

That all depends on what your current subnet is and what you want to accomplish with your new subnet - how many hosts; is this subnet for routing purposes (i.e., to segregate a segment of your network), or do you just need more host IPs available on your network, etc.

Add to that -
Once you define what you need from the new subnet, how will you access it (route to it), manage DHCP (assuming it's needed), site setup (in AD), etc.

If you question is solely along the lines of Win2k3 and what it needs -
DHCP needs to be on both networks assuming you use DHCP.  This can be done with one multihomed server (not recommended) or with multiple scopes with a router forwarding DHCP broadcast from the new subnet to the existing server - OR - with a second DHCP on the new subnet.

Assuming you want traffic to go between subnets you need a router or switches that can route between the subnets.

AD Sites and Services should have the new subnet added and serviced as local.

Many more possible items but as hypercat states, we need to know more.

We have three subnets.  Two of them are for another building across town. and one of them is here at our corporate office.  We are running out of IP's here, especially DHCP.  We have a point to point with Cisco routers routing the IP's between the different buildings.  All servers are at our building so most computers just get to them by using the switches.  We have a firewall installed for outside traffic.

We would just like to create a fourth subnet for extra addresses.  

Thank you.

You could just change your subnet mask if it's not too big and over broadcasting already.  Still would need to adjust your routers and statics though, may be more work than adding a subnet and letting the default gateway handle routing between them.

So if you already know how to have multiple subnets, what are you asking for help on?
How to physicaly split your networks?  VLAN switching?

These subnets were setup long before I was here.  I am looking on how to setup a new one.  I could go with VLAN which seems easier but like I said, there are multiple subnets setup well before my time here.  I am looking for instructions on how to setup a brand new subnet on an existing domain.

I don't think this can be properly covered in a forum, I would recommend contacting a consultant to map out a plan.

That said, if you choose adding a subnet as the solution...

There are a few reasons for using a VLAN, in your case lets you use it to allow separation of traffic and broadcasts.  Assuming your current switch setup allows it, you can assign ports to VLANs according to what subnet you want the workstation on.  Since you can pick what port goes to what subnet, you could split PCs within the same room on the same switch.  Your core router (default gateway) would need another port with an IP from the new subnet assigned to it - this will be the default gateway for the new subnet.  Switches can also function as routers between subnets but I don't know what yours are capable of.  Without VLAN you would have to physically split your network with different switches for each subnet.

As mentioned earlier, you need to configure DHCP for the new subnet and a site.

There are a lot of other possible issues but you would need to sit down and map out your network to go over possible route issues, static IP changes, etc.

We have the new subnet mapped out and picked.  All we need to do is implement it on our network.  I went into the AD Sites and Services and no subnets are in there.  In the DHCP and DNS records all three subnets are in there.  We are picking an ip and giving it a mask of 255.255.255.0 /24.  That will give us 254 addresses to use.  We are trying to keep it simple.

Thank you.

We have routes setup in our firewall and our point to point routers.  I will need to figure out how to add the routes to these.  Once a DHCP scope is established and the routes are setup, then will the new subnet work?