qualifing the tables with the appropriate schema

Hello,
Given the code/script attached, how would one qualify the tables.  How would one identify the "appropriate schmea."  Obviously I am new at this, I'll have to do some studying this weekend.

select    cl.home_phone, cl.src_acct_nbr, cl.hsd_prod_flg, cl.video_prod_flg, cl.tele_prod_flg, nc.network_id,
          nc.node, nd.device_name, cl.node, cl.customer_id 
      from 
         network_device nd,  
         network_config nc,    
         customer_lkup cl       
      where 
        nd.device_type in ('SOS')        
        and nd.device_name like 'n03.a.com.den.net'       
        and nd.device_name = nc.sos
        and nc.network_id = cl.network_id 
        and cl.last_actv_dt > cl.last_disconnect_dt    
        and nd.system_id = '007'

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