A software bug caused an online mobile services blackout last Friday, affecting up to 50,000 customers of 3Hong Kong, the telecommunications operator told an industry watchdog yesterday. In an incident report submitted to the Office of the Communications Authority, the subsidiary of Hutchison Telecoms said a bug disrupted the allocation of IP addresses to users' handsets, an essential step to establishing internet access. The bug reduced the number of unallocated IP addresses in the pool to below normal, resulting in some users failing to get an IP address. Phone calls and SMS services were not affected during the two-hour blackout. The firm said it was seeking a permanent fix for the software problem as soon as possible. Charles Mok, founding chairman of the Internet Society, said there were not enough IP addresses to cover all electronic devices in the city. Consequently, operators will only allocate an IP address to a handset when its user attempts to go on the web. It is impossible for a user to access the net without an address. In a similar case, SmarTone said it was willing to pay a fine to Ofca for a major service disruption three months ago, but would not offer compensation to any of the 243,000 users affected. Mobile phone services at a quarter of SmarTone's cell sites were disrupted by a blackout on April 9, preventing users on Hong Kong Island and at some MTR stations in Kowloon from making calls or accessing the internet for at least two hours. If SmarTone was found to have breached its licence conditions, it would face a fine of up to HK$200,000 for a first offence. Ofca has yet to decide whether SmarTone will be fined.