Getting more IP's for business

you could set up a server to do ip routing. fairly easy. two nics and really any decent box will do

You don't want to make this more elaborate than you really need to.
So, unless you know that you have lots of traffic already then I would expand like this:

Change the subnet mask:
From: 255.255.255.0 (that's what it is, right?)
To: 255.255.254.0

This will change the number of addresses in the subnet from 256 (254 usable) to 512 (510 usable).

I would make the change in the gateway router first.
Then on the DHCP server if it's different.
Then on any devices with static IP (i.e. manually entered) IP addresses and subnet masks.

Fortunately, 192.168.100.0 /24 or 255.255.255.0 is also the base address for
192.168.100.0 /23 or 255.255.254.0.  
This means that there may be no noticealbe disruption at all as you make the changes.
All of the present IP addresses can stay the same / with the new mask.

If you're relying heavily on DHCP then you will likely want to increase the DHCP address range or, even, move it.  For example:
From 192.168.100.128 - 252
To: 192.168.101.2 - 252
That's up to you......

You don't need to add any NICs to do this.
If, after it's done, you discover that your actual network traffic is just too much for the number of devices then you can consider split subnets, VLANs, etc. etc.
BUT:
First of all you didn't say that traffic was the issue.  If not, then immediately after the change there will be no difference at all.  After all, it's the number of active devices and their level of activity that determines the traffic and NOT the number of available addresses.
Second, this approach gives you more addresses rather directly and immediately without further fussing around.

I warn you that some will say that this approach is inadvisable because you might end up with traffic issues (some day).  It wouldn't hurt for you to be prepared for dealing with the traffic issue even if you don't have to go there....  And, you will have time to contemplate that.

The above method is the least painful for the moment, but if you are up to redo your ip scheme you could supernet...

What else will I need to purchase, in terms of equipment, hardware, software, and services?  Thanks...

"What else will I need to purchase, in terms of equipment, hardware, software, and services?  Thanks... "If you're using Windows, you wouldn't have to purchase anything.  Just change your existing configuration.

http://technet.microsoft.com/en-us/network/bb643151.aspx

http://www.windowsnetworking.com/articles-tutorials/windows-server-2008/How-to-Install-Configure-Windows-Server-2008-DHCP-Server.html

So I can't just change the subnet mask to 255.255.254.0 and get double the IP's, I still have to buy a new server as well?

Sorry my understanding of how this all works is still a little unclear.

Yes, you can change the subnet mask.  You don't need to buy anything.

I presume the links to Windows Server-based arrangements were intended to point to the instructions IF you have a Windows Server-based system.  But, if you don't, then the focus of activity would be elsewhere.  I wasn't specific about it really. Your "DHCP server" could be a Windows Server or it couid be a commodity router or anything in between.  Either way, no worries as long as you know which device is providing addresses via DHCP and how to change the subnet mask there.....
If you're not using DHCP at all then it's not an issue.  But you still have to change the subnet mask in the gateway device (and all the other devices that won't be handled via DHCP).

As noted, you needn't buy anything new, just reconfigure what you have.

If you're running Windows (and you still haven't said) I don't think you can just change the subnet mask.  You will probably have to create a new DHCP scope that specifies the new subnet mask.  Once that's set up and configured, you can delete the old scope and carry on.

Well we still need both scopes as we have a bunch of static IP's that are on the 100.x.  so if I change the subnet mask and then create a new scope for the 101.x IP's, what happens to the ones in the 100 scope?

If you create a new scope with /23 instead of /24, the new scope will encompass the old one.  the old machines can keep their existing static IPs and you can create reservations in the new scope for those machines.

And by the way this is Windows Server 2008 R2.

Is there a benefit to doing it this way as opposed to any other ways? Or is this best practice?

You guys are freaking amazing. Thanks so much.

The benefit of doing it this way is that there are fewer steps necessary in device settings, etc.  Thus, fewer opportunities for errors; typos or understanding of a new archtecture that may fail.  It's less work and lowest risk.

The disadvantage is that the subnet may be "too large" for your needs (traffic).  But unless you plan to increase things in a big jump, it should not be an issue in the near term.

Other advantages/disadvantages are really outside the scope of the question.  For example, what if you *want* to separate subnets for some reason?

Thanks for the points!