iptables: cannot ping source ip address

Hi!

I want to setup an linux apache server which does allow access only to
certain ip addresses. For this I want to use iptables to drop all the
packages which are not from the allowed ip adresses.

Unfortunately the server with the source ip-addresses is setup in a way
that it doesn't allow ping commands (is filtered by the source server).
So when I start up iptables I get the message that

host/network `x.x.x.x' not found

But I'm sure that this ip-address exists - I see it i.e. in the log of
apache.

Is there any way to tell iptables just to set up this filter rule and
not to care if the ip-address can be found?

thanks for any help

Daniel Achermann