Networking :: Setup Pxe Server Routed Via Standard ASDL Router

I have searched google, but can't really get the hang of setting this up.Most howtos are setting up a DHCP and PXE on the same box. At the present my DHCP is done by my router and I want to set up the PXE server on my main PC.My router can re-direct traffic types based on ports (UDP or TCP) to an IP but can't do the bit about directing the pxe loader name (and I really don't want to ssh into it and start messing about there)If I redirect the port (what ever it may be, help required here) how would I go about setting up the Ubuntu PC to do the load and pass back to the PC trying to PXEboot?