[ppml] IPv6 assignment - proposal for change to nrpm

William Herrin arin-contact at dirtside.com
Sun Oct 21 22:52:06 EDT 2007

On 10/21/07, briand at ca.afilias.info <briand at ca.afilias.info> wrote:
> What, if anything, do you believe forms a *requirement* that a Legacy
> /24 cannot use IPv6, unless they receive a /64?


I think you have mistaken me for Keith Medcalf whose assertion that
legacy /24 holders should receive a /64 confuses me as well. They
should receive a /48 IMHO, just like every other PI assignment. That's
not an engineering requirement but it is a best practice.

> (1) The IETF does not recommend use of specific prefix lengths;

That may come as a surprise to your compatriots in the IETF. The
recommend netmask of exactly /64 on any broadcast-capable LAN has come
through loud and clear. So clearly, in fact, that a number of folks
are surprised to learn its a recommendation and not a requirement.

> (3.2) Specifically, Link Local addresses are constructed as right-aligned
>       Interface Identifier, zero-padded, bitwise "OR"-ed with 0xFE80::.
>       And, the Link Local address does *not* have a prefix length (!!)

$ ifconfig eth0
eth0      Link encap:Ethernet  HWaddr 00:04:23:AC:CA:32
          inet6 addr: fe80::204:23ff:feac:ca32/64 Scope:Link

Note the "/64" on the autoconfigured link local address.

> The reason I encourage use of a smaller prefix size than /64, is that the
> overall scalability of *all* of the ISPs who have full routing tables,
> is dramatically affected by the likelyhood of *any* ISPs getting more than
> one PA block for giving out to customers.

This is the purpose of holding the nearby space in reserve so that the
ISP can simply shorten the netmask upon returning for additional
addresses. With "sparse allocation" the RIR can further maximize the
probability of meeting the ISP's second and subsequent requests for
address space from within a block that doesn't require an additional
prefix in the BGP table.

None of which has anything to do with assigning user prefixes longer than /64.

Bill Herrin

William D. Herrin                  herrin at dirtside.com  bill at herrin.us
3005 Crane Dr.                        Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004

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