I would definately get a hardware firewall, especially with Windows 98, considering it is end of life and there are no more security updates available for it. You've specified that you have been given an IP address range of 128.174.7.0 to 128.174.7.64. I'm going to assume they they broke this down into a /26? (although that still doesn't make sense, and it's kind of scary if they broke it off without subnetting it, especially without a firewall).
1. You need to find out what the subnet mask is for your IP range.
2. You need to find out the IP of your gateway.
128.174.7.0 is a network address, and non-addressable. So you need to start with 128.174.7.1 (unless this is your gateway). I generally like to leave the first 10 or so IPs open for network/firewall equipment, but it doesn't look like you have a lot of need for that here. ASSUMING your gateway (which would be your router or firewall) is 128.174.7.1, let's start with 128.174.7.5. This leaves 4 addresses available in case you need to add anything additional (this is by no means mandatory, just a personal preference to not have network/servers scattered throughout your IP pool). If this were a /26, then your broadcast would be 128.174.7.63 (we don't know this for sure, but let's just assume).
Taking away .1-.4, giving .5 to the server, this gives us 57 IPs to work with.
Create yourself a spreadsheet, number the first column of cells 128.174.7.1-128.174.7.62, make a second column for the description. Add your gateway at .1, add your sever at .5.
At this point, map out each of your IPs.
When you are ready for the move, assign the new IP to the server along with your gateway and DNS. Assign each of your 98 systems with the IP Since this is windows 98 with NT 4.0, you will also need to change the WINS address on each 98 system.
Without knowing what else is going on, this is about as detailed as I can get with a general plan.
