Bitten by IPv6

We've been getting comments about a story that appeared in GCN a few months ago on how many addresses are actually available with IPv6. In a Feb. 5, 2007, article ' 'DOD to allocate its IPv6 addresses' ' we mentioned that the Defense Department has acquired a /16 Block address, which we estimated to provide 247 billion IP addresses. Several readers have pointed out that number is wrong, wrong, wrong! It is far too low. And, in fact, even if we had gotten it right, the number would be pointless.

And they were right on both counts, we subsequently found.

It is best to start from the beginning. The first 64 bits of every IPv6 address are reserved for network routing ' so no organization can play with those numbers. A /16 means that the recipient gets approximately 16 of the leftover 64 bits for externally reachable routing ' so outside parties know where to look on the Internet for this organization. The other 48 bits can be used for subnets. Each bit could hold one of two values ' 0 or 1 ' so the total number of unique addresses within the 48-bit address space given to DOD would be 2 to the 48th power, or approximately 281.47 trillion.

So not only were we wrong, we were wrong by at least three orders of magnitude. And that's just the start. Now, a /16 holder has 281 trillion subnet addresses, meaning each of those addresses could be a gateway to an internal network. And each of these subnets can have a full 64 bits for its own addressing scheme, thanks to IPv6's auto-configuration capabilities. As a result, 'each subnet [has] a potential address space of approximately 18.45 quintillion hosts in the 64-bit host space,' said one government information technology specialist. This number is slightly whimsical however. 'Since a [local-area network] will have many fewer hosts, people are generally not concerned with the utilization of the number of hosts on the LAN,' another writer noted.

So there you go: A /16 (pronounced 'slash 16') gets DOD 281 trillion network addresses and ' theoretically ' 18.45 quintillion host addresses. It took us a long time to figure this out, so please don't send us any advanced questions on IPv6 numbering. We'll only direct you to the Internet Engineering Task Force documents on numbering, RFC2373 (GCN.com/791). Thanks to all those who e-mailed.

About the Author

Joab Jackson is the senior technology editor for Government Computer News.

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