Subdomains on the same network

[WeeWoh]

I have some old Compaq Deskpro p3 500MHz servers that I am using to make a website. One is running Ubuntu Server and another is running Debian. The Ubuntu Server is a LAMP Server while the Debian Server is just running php.

The domain is jjtcomputing.co.uk and I am on a dynamic ip address.

The Debian server is the main server that uses the jjtcomputing.co.uk. However, the forum is hosted on the Ubuntu server. They are both on the same network. I want to set up a sub domain forum.jjtcomputing.co.uk and that request to be sent to the Ubuntu Server while jjtcomputing.co.uk is sent to the Debian server. How might I do this?

I have a spare Deskpro (possibly 2) to muck around with, and need to be using Linux.

P.S. the Debian Server is 192.168.0.3 on the internal network and the Ubuntu Server is 192.168.0.2. I have enough network cards to put them behind a gateway (the spare Deskpro) and possibly a switch coming to make it simpler.