ISPs have been worried to learn that BT's ongoing multibillion pound "21CN" network upgrade won't offer native support for IPv6, the networking protocol that it's planned will form the bedrock of a rapidly expanding 21st century internet.

The ommission surfaced when business provider AAISP, one of a select group of resellers currently able to offer ADSL2+ via BT's 21CN network, asked BT to fix a bug in Cisco equipment that was affecting subscribers using IPv6. BT Wholesale replied: "BT currently supports IPv4 on it's broadband products and does not support IPv6."

It's still possible for ISPs to offer customers IPv6 services, but they'll have to use tunneling, which adds to their overheads.

AAISP said: "Tunneled IPv6 works in all cases, and native IPv6 works in some cases where this bug is not affecting it. In some ways the issue is more commercial than technical - IPv6 mostly works, but BT appear to be saying that if it stops working then they won't fix it."

IPv6 was defined in 1996 when it became clear that the current IPv4 protocol would eventually run out of addresses. Recent estimates say IPv4 could be exhausted in 2011.

The delay-hit rollout of 21CN, which will shift all of the national telecoms backbone to packet switched networking, is currently scheduled for completion in 2012. BT was unavailable for comment this morning. ®