[CentOS] Problem with CentOS server: am I running a firewall?

I recently tried to swap server from an ancient Asus PIII machine
running Fedora-8 to a Dell PowerEdge T105 running Centos-5.1 .

Unfortunately, I have not been able to set it up
to allow local machines to access the internet.

I can access the internet directly from the server
(which connects to an ADSL modem)
but not from a laptop connected to the server 
through an ethernet hub or WiFi router.

I can ssh into the server from the laptop,
but cannot ping anything beyond the server.

It is as though there were a firewall running on the server
which will not allow packets on the LAN to pass to the internet.

This is probably a very ignorant question,
but is there such a default firewall running under CentOS?

I've gone back to my old server temporarily,
but would prefer to sort out the new one if I could.

Any advice or suggestions gratefully received.