Hello.
Here is my proble :
I have a small internal LAN of 3 pc, and they share one internet connection, one of these pcs is router. My problem is that i have an irc server runing on the pc which has internal ip 192.168.1.10 on port 6667 , my router has ip for internal network 192.168.1.1, and for example 1.1.1.1 for external so i must dnat to this internal irc server. i write a chain in my firewall script.


$IPTABLES -t nat PREROUTING -i $EXTIF -p tcp --dport 6667 -j dnat -to--destination 192.168.1.10

but still people cant connect my server here is the module list which loads with my firewall script


echo -en "ip_tables "
$MODPROBE ip_tables


echo -en "iptable_filter "
$MODPROBE iptable_filter

echo -en "ip_conntrack "
$MODPROBE ip_conntrack


echo -en "ip_conntrack_ftp "
$MODPROBE ip_conntrack_ftp

echo -en "ipt_mark "
$MODPROBE ipt_mark


echo -en "ipt_multiport\n"
$MODPROBE ipt_multiport


echo -en "ipt_state "
$MODPROBE ipt_state

echo -en "ipt_owner "
$MODPROBE ipt_owner

filtering at all.
echo -en "iptable_filter "
$MODPROBE iptable_filter

echo -en "ipt_REJECT "
$MODPROBE ipt_REJECT

echo -en "ip_conntrack_irc "
$MODPROBE ip_conntrack_irc


echo -en "iptable_nat\n"
$MODPROBE iptable_nat


echo -en "ip_nat_ftp "
$MODPROBE ip_nat_ftp


echo -e "ip_nat_irc "
$MODPROBE ip_nat_irc

Your problem looks a bit like the one I had some time ago.
maybe is your iptables blocking other things, or the irc server is not accepting everything.
run
IPTABLES -L on your router, to see the current status of your ruleset. If it is way too much, you can reset it with:
IPTABLES -F
IPTABLES -t nat -F
IPTABLES -t mangle -F

you can also try to check with nmap from a computer on the internet if port 6667 is blocked by defaukt.
try to run an irc server on your router see if that works (only for a moment)

also loosen it a bit up like accepting UDP
$IPTABLES -t nat -A PREROUTING -p udp --dport 6665:6669 -j DNAT --to 10.10.10.10
$IPTABLES -t nat -A PREROUTING -p tcp --dport 6665:6669 -j DNAT --to 10.10.10.10

maybe enable masquerading and accept forwarding, I don't know if it is disabled by default.
$IPTABLES -t nat -A POSTROUTING -o $EXTIF -j MASQUERADE
$IPTABLES -A FORWARD -i $LAN_IF -j ACCEPT

I hope this will help you a little.