Subtracting two tables

I have a table "Supplier_Entity" and it has two colums Seller_id and Supplier_Name, there is this another table out_of_network_suppliers having same columns.  Supplier_Entity= out_of_network_suppliers+ in_network_suppliers. "in_network_supplier" is not a table, how do I get the data for in_network_supplier. Basically how do I remove the out_of_network_suppliers data from Supplier_entity in order to get desired data.