Your installation server needs to be configured to serve the TFTP protocol. Since the client upon booting doesn't know anything about
where or
what TFTP server it is to use, PXE requires the client to broadcast its request, & whatever server responds is the server used. This is the same model used by DHCP. If the server on the same subnet does not respond, it is a question of whether the intervening routers are configured to pass broadcast messages from one subnet to another. If you have control over all of the intervening routers, you may be able to separate the client(s) from the TFTP server, but if you are doing this over the Internet, remember that the protocol specifies that the
"first server" responds to the request. If this
isn't your server, this is a problem.
For more information, study
Section 6.10 of the official FAQ, & the
tftp(1),
tftpd(8),
pxeboot(8) manpages.
In addition, you will need to study the
boot(8) manpage for information on
boot.conf as this file will be required on the TFTP server as well.
You may also want to study Wikipedia's introductory article on
PXE. A PDF of Intel's specification can be found at the bottom of the article.