nat/masquerade with kernel 2.6.x

["Antonio Pérez" <antonio.perez02@alu.umh.es>]

Hello,
I hava a local network with private directions, and a adsl conection. I 
want to do masquerade. Then I do:
    echo 1 > /proc/sys/net/ipv4/ip_forward
    iptables --table nat --append POSTROUTING --out-interface eth1 -j 
MASQUERADE
where eth1 is the interface conected to internet. This work perfectly 
with the kernels 2.4.x.
But one week ago I installed the kernel 2.6.8 and when I do :
    echo 1 > /proc/sys/net/ipv4/ip_forward
    iptables --table nat --append POSTROUTING --out-interface 
$ifc_internet -j MASQUERADE
the hosts of the internal network can do ping to internet, and this is 
normal, but they can not open any web or conection the msn , they
only can do ping. This is very stranger. I try the kernels 
2.6.7,2.6.7,2.6.8 and 2.6.9 and they do no work.
Can somebody help me, please?
Sorry for my bad english.