how can i monitor all packets coming in and going out of my pc?
also how can i enable a port using iptables.

It would depend upon what packages you have installed on your system. A good place to start for packet monitoring is www.ethereal.com. If you are using Red Hat the package may already be installed, so type ethereal at the prompt. If you prefer command line options try running ethereal -h or man ethereal .

You may also want to try Cricket if you are looking at traffic trends: http://cricket.sourceforge.net/

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** IP TABLES **
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Say you want to enable outgoing FTP from iptables:

iptables -A INPUT -i $IFACE -p tcp --sport 21 -m state --state ESTABLISHED -j ACCEPT
iptables -A OUTPUT -o $IFACE -p tcp --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT

where IFACE="eth0,eth1,...etc" as you see appropriate.

You can also try:

iptables -vnL

as iptables keeps track of traffic.

There's also tcpdump, which most distros include

I very much like ethereal and reccomend it.