LCNs in Australia may have one, two or three digits. Each network is allocated LCNs starting with a certain prefix - for instance, all metropolitan Nine Network services use LCNs beginning with the digit '9'. Generally, but not always, the single-digit LCN is allocated to the primary SD service (Network Ten's sub-channel One being the main exception). LCNs need not be contiguous, and a channel may be identified by more than one LCN. For instance, ABC Television's primary ABC service is allocated LCNs 2 and 21; the latter allows it to be easily accessed amongst other ABC services which lie in the 21–24 range.