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"A value which indicates the set of services that this
           entity may potentially offer.  The value is a sum.

           This sum initially takes the value zero. Then, for
           each layer, L, in the range 1 through 7, that this node
           performs transactions for, 2 raised to (L - 1) is added
           to the sum.  For example, a node which performs only   
           routing functions would have a value of 4 (2^(3-1)).   
           In contrast, a node which is a host offering application
           services would have a value of 72 (2^(4-1) + 2^(7-1)).  
           Note that in the context of the Internet suite of
           protocols, values should be calculated accordingly:

                layer      functionality
                  1        physical (e.g., repeaters)
                  2        datalink/subnetwork (e.g., bridges)
                  3        internet (e.g., supports the IP)   
                  4        end-to-end  (e.g., supports the TCP)
                  7        applications (e.g., supports the SMTP)

           For systems including OSI protocols, layers 5 and 6
           may also be counted."