VPN through NATed devives to pix

I am having some problems connecting to a pix firewall vpn connection
>using
>the cisco client ver 4.6 when the clients are behind a nat device to the =
internet.( LINUX FIREWALL IP TABLES)  Same configuration worked well with 
secure remote client and checkpoint firewall
>Is there a way to let them connect without giving them a routable ip
>address
>or modifying there routers at all?  Have any of you ever had to get =
past
>this problem?  Is it possible to get past this problem?  I am new to =
pix
>but
>I have done some research.  It seems that we need version 6.3 of the =
OS

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