A method and apparatus are provided for fault location in a loop network (100, 200, 400). The network system having a host port (214) for supplying and receiving data and a plurality of successively connected ports (201, 202, 203, 204, 205) through which data from the host port (214) is transferred. A counter (122) for each port records data transfers in which the amount of data received at a destination port is less than an expected amount of data. When a transfer with less than the expected amount of data is identified for a data flow between a sending port (201) and a destination port (214), the counters are incremented for each port (202, 203, 204, 205, 214) after the sending port up to and including the destination port. Analysing means determines a fault location in the network system from the distribution of counts in the counters (122).