iptables --set-class

Use the netfilter for classification.

iptables -t mangle -A FORWARD -m mac --mac-source 00:00:00:00:00:01 -j CLASSIFY --set-class 1:11

The --set-class only work with "mangle" table and FORWARD chain with iptables (v1.3.5).
(Tested on kernel 2.6.18, RHEL 5)

Other combinations give this stupid error message:
iptables: Unknown error 18446744073709551615