Why does access to kickstart server:ks.cfg using http fail on Red Hat Enterprise Linux 6 if the client is in different subnet?

Issue

• Have a working kickstart server on Red Hat Enterprise Linux 5.8 and can start from other VLAN during kickstart (no DHCPD service is running). \
• The new kickstart server is on Red Hat Enterprise Linux 6.4, and seems it only can build client if it is on the same VLAN, is there anything required to get done to have the same function as 5.8.
• SElinux and iptables are disabled as well.