|I think you are missing something very important. NAT, where is your NAT. Most networks are setup with one or more Public IPs (Static or Dynamic) which are NATed to a Private network. This gives you the ability to hook up many computers up to 254 if you use /24 like said above. If you want to go beyond 254 computers then you subnet using the subnet mask. |
What you have said above seems to state that you are not using NAT at all and you are assigning the Public IPs direct to the computers. If that is the case then you ISP will give you a CIDR block which will be like /30 (CIDR Notation) or 255.255.255.254 Subnet. This gives you 4 Public IPs.
"The fact is that my ISP allocated me an ip from a totally different network that seems private to. It is a /25."
If you ISP gave you this then chances are they are not private. Look at the first two octets If they are either one of these...
172.16.x.x - 172.31.x.x
Then they are private, if not then they are Public. The /25 says that you have a CIDR block of 128 address assigned to you. This means that if your first address is 126.96.36.199 then you can use address up to 188.8.131.52.
So you put your Static Private IP and the Subnet Mask settinging into your NIC and you are on the Internet. Then you either get your ISP or third party like GoDaddy to register your IP on the Public DNSs. This will give you a www.xxx.com, .net, .gov or what ever the DNS you registered with. This way people can connect to you servers with the DNS name and not the IP.
Now it sounds like you are switching from Static to Dynamic IPs. If you do so then you will not be able to do any web hosting from your servers unless you get a third party proxy which will install an app on your servers and phone home to the static servers what your IP is. Its better to have static IPs from your ISP if you want to do this.
That is the quick and dirty on networking and now you can see why every one is confused because what you have said does not seem to fit. Yes there are more details than what I listed here and I am sure I am going to get flamed but you need to give us more to go on.