Iptables setup

After setup XDMCP protocol in Linux, It's necessary to Activate XDMCP port(default 177) under iptables.
There are three system chain in configure, INPUT, OUTPUT and FORWARD. And input and forward are refer to RH-...-INPUT(short as RH) which is define by redhat users.
And all rules are based on RH. "-A RH -p udp -m udp --dports 0:1023 -j REJECT"(1) means If packet with udp protocol to 0:1023 port is reject(option Drop is similar to REJECT but without Answer).
As these, I add "-A RH -p udp -m udp --dports 177 -j ACCEPT"(2) at the bottom, means to actives 177 udp port. But it doesn't work!
Try again and again, I find the correct way is to add (2) before (1).
Nice try