Firewall/NAT

I have 13 IP address and I'm wondering how I can setup two FTP servers.  Right now I forwarded port 21 to go to the right server 192.168.2.10, but I can only use the port once.  I've setup 1:1 NAT so I can now type 2.0.0.0 or 2.0.0.10 (external IP) to go to my ftp server, but I would just like it to be 2.0.0.10.  Now we want to setup a second ftp server 2.0.0.11, but can't because port 21 is being used.  In summary, what I want to do is the following:

2.0.0.0   (port 21) won't go to anything
2.0.0.10 (port 21) will go to 192.168.2.10
2.0.0.11 (port 21) will go to 192.168.2.11

How can I accomplish this?  Thanks

So it seems like the way to do this would be server NAT.  I went ahead and did this and everything worked.  However, by doing server NAT I lose the IP address.  For example, before with 1:1 I could go to whatismyip.org and get 2.0.0.10, but now with server NAT I just get the 2.0.0.0.  Is there any way around this so I can keep my 2.0.0.10?

Yes, you need Server or 1:1 NAT with multiple public IP's if you need to open a single port to multiple internal machines.

Add your additional public IP's in proxy ARP and then either configure 1:1 or Server NAT as desired.