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Andrews & Arnold (A&A), one of only a few ISPs in the UK providing IPv6, discovered the shortcoming when some of its customers began to see problems with their internet connections.

Those problems were tracked down to a bug in the 21CN hardware, which, the ISP claims, BT declines to fix.

“The whole issue has come about because of a bug in Cisco equipment which BT use which is affecting use of IPv6 for some of A&A’s customers,” the ISP says. “It only affects some of BT's network. Even though we believe this bug was identified and fixed by Cisco a long time ago, BT appear to be refusing to rectify the problem, preferring to simply say

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they do not support IPv6.”

There is nothing inherent in 21CN that prevents it supporting IPv6. On the contrary, says A&A, the specification is clear that it allows ISPs to pass packets to and from their end users using point to point protocol (PPP) which would allow ISPs to offer IPv6.

IPv6 is working in some instances, where the bug is not present. Elsewhere A&A is bypassing the problem by tunnelling over IPv4, but that comes with overheads that affect performance.

BT’s response was short and to the point.

“I can confirm that BT currently supports IPv4 on it's Broadband products and does not support IPv6,” it said. “If you believe this will be a requirement for you going forward I would ask you to submit a formal Statement of Requirements, to enable BT to formally evaluate the same.”

At the moment this is an issue for A&A, its customers and other IPv6 users, but soon it will be an issue for everyone. Under the current IPv4 system, all possible internet addresses are likely to be used up by the end of 2011, if not earlier. By supporting longer addresses, IPv6 solves this problem. It has capacity enough to provide unique addresses for each person on planet, with many billions to spare.