Abstract: A tester for testing communication links in a network which carries data frames between communications ports having respective station addresses includes a plurality of communications ports and a test data generator for generating test data frames to be transmitted via the communications ports. A store holds a plurality of sets of predefined station addresses to be associated with the communications ports. A selector receives an indication of user selection of one of a plurality of test modes of the tester, and selects in accordance with that indication a respective one of the sets of predefined station addresses for association with the communications ports. The selector may also receive an indication of user selection of one of several test modes, and select a respective mode of operation of the communications ports and of the test data generator.