Patrick,
I'm not a 100% sure of the algorithm used to determine what subnet is assigned to which WCCP bucket. However, I do know it involves an XOR of various L3 and L4 header fields in the packet.
To view the how the calculation has been performed you can run the hidden IOS command
show ip wccp hash <dst-ip> <src-ip> <dst-port> <src-port>
Router# show ip wccp 61 hash 0.0.0.0 10.88.81.10 0 0
WCCP hash information for:
Primary Hash: Src IP: 10.88.81.10
Bucket: 9
WCCP Client: 10.88.81.12
Router#
Hope this helps,
Mike Korenbaum
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Cisco WAAS PDI Help Desk
http://www.cisco.com/go/pdihelpdesk