I am designing a computer network for a public high school and came up with the following division of responsibilities between the servers:
In the LAN:
Active Directory Domain Controller. The DC will serve approx. 130 devices in the LAN. DNS, DHCP and RADIUS run here.
File server. User profiles, data shares, mail.
Application server. SQL server and several other programs, such as IP camera recorder, building access control, lunch ordering system, etc.
In the DMZ:
Internet gateway. Provides access to the Internet. Runs Exchange, IIS and a web-caching proxy.
Web server. Debian GNU/Linux hosting several websites and webmail.
This is five servers. Although I can acquire the necessary hardware, it seems like a lot. And for some reason, I do not like having servers virtualized.
If I use this design, I would like to take advantage of it and run another AD DC on the file server, just in case something goes wrong with the first server. But user profiles are stored on the file server, so if the file server goes down, everything goes down.
So maybe I could move the data onto a disk array attached to both the AD DC and the file server. Both of them would serve as a DC and file server. I could even run secondary DNS, backup DHCP and backup RADIUS on the second server thus making all services redundant for a little more configuration.
Half of the computers in the LAN could have the first server configured as the primary DC/DHCP/DNS, the other half could use the other server as the primary to balance loads.
SQL server is absolutely critical on the Application server. I could run a backup SQL server on the first DC. The other apps on the application server are not as critical.
Two machines are in the DMZ. Unfortunately we need IIS server in the DMZ. Otherwise a single GNU/Linux server would do. (I would use postfix instead of Exchange Server.)
I do not have much experience with the design of computer networks (except for very small networks or separate servers) and would like to know whether or not is my design reasonable and whether there are any inherent flaws.
If there is a good book on the subject, please let me know.