[Click] Where are packet types used by Classifier element defined?

[betamaz]

Hi,

When using the Classifier element where are the offset/value(s) defined? I 
found that include/clicknet/ether.h specifies the packet type using the 
values:

#define ETHERTYPE_IP 0x0800
#define ETHERTYPE_ARP 0x0806
#define ETHERTYPE_8021Q 0x8100
#define ETHERTYPE_IP6 0x86DD

But, I dont know how the offset is determined. For example in Classifier 
documentation we have 
Classifier(12/0806 20/0001,
12/0806 20/0002,
12/0800,
-);


So 0806 stands for ARP packets and im guessing the 12 is defined in 
include/clicknet/ip.h. 
Can someone explain where the combo 20/0001 and 20/0002 which speciy arp 
request/replies are defined?

Thanks a bunch,
Bita.