determining if IP is from intranet or internet

[Rob Willis]

I am capturing packets on the segment where a web server is located. 
Requests can come from any of several internal subnets or from external
internet sites.  I need to determine if the IP originated from the local
domain or from an external domain.  I do not know ahead of time all of
the network addresses for the internal subnets.  The internal subnet is
made up of several class C networks.  Is there a way to request a list
of valid subnets for the local domain from the DNS server.  I could then
compare the captured IP's against the list of known internal subnets to
see if the http request originated internally or externally.  Or is
there a better way to do this check?

Can anyone help?
-Rob


Rob Willis
CACI Products 
robw at caciasl.com

PS  please copy all responses to my email