It's not clear whether the same updates will be made available for older hardware but, considering the amount of work involved and the fact that companies will receive no income from doing so, it seems unlikely. According to Cisco, there are also practical limitations, such as a lack of memory in older devices.

Of the routers that already claim to support IPv6, many of the implementations are buggy and . This even includes those certified as IPv6-compliant by the .

Security experts are also warning that the sheer number of addresses available via IPv6 will make spam blocking impossible. Currently, the most effective anti-spam technique is to simply blacklist certain internet addresses that are known to send spam messages. This works because it's difficult for spammers to acquire additional IP addresses. However, because IPv6 has such a huge range of available addresses, spammers could easily acquire thousands if not millions of them. Each individual spam message could come from a unique IPv6 address, making accurate blocking impossible.

Because of this, there's talk of sticking to IPv4 for mail servers. Any incoming mail from untrusted IPv6-based mail servers could simply be bounced back. Until a reliable technology is invented, mail servers may never break free of IPv4-based addressing.

It isn't just spam that relies on blacklisting. Preventing denial of service (DoS) attacks, click fraud and preventing manipulation of search engines could also be hit, .