Simple table/chain question #8

[Planet X]

Ok, this has been bugging me.

Firewall eth0=192.168.0.1
            PPP0=x.x.x.x
Workstation eth0=192.168.0.8

#8 If a TCP packet is going to my workstation, how does the firewall
        _know_ to forward that packet to the workstation since the
destination
        would be the PPP0 IP# when coming from the internet? We can
        pretend its an ICQ packet using TCP going to the workstation.

#8a  Does the firewall know to forward it based on TCP Sequence #? Or
        does it route based on the IP Ident number? IP headers are much
larger
        and contain a lot more info....In otherwords, a simple ICQ message
        packet might be TCP (lets just say it is for argument sake), but its
        encapsulated in an IP packet, right? So, the firewall forwards my
ICQ
        TCP packet based on....what? Since the destination is the same as
         the PPP0 IP# for my entire LAN due to NATing, right??


Please let me know if you need clarification- I did not word this very well
at all. I can use my NEWBIE to GURU translation booklet if you need me
too. Thank you for your patience and I truly appreciate it!

Joel