# File lib/eui.rb, line 188
    def ei(options=nil)
        known_args = [:Delimiter]
        delimiter = '-'

        if (options)
            if (!options.kind_of? Hash)
                raise ArgumentError, "Expected Hash, but #{options.class} provided."
            end
            NetAddr.validate_args(options.keys,known_args)

            if (options.has_key?(:Delimiter))
                if (options[:Delimiter] == ':')
                    delimiter = options[:Delimiter]
                end
            end
        end

        if ( self.kind_of?(NetAddr::EUI48) )
            ei = octets[3..5].join(delimiter)
        else
            ei = octets[3..7].join(delimiter)
        end

        return(ei)
    end