Private use of non-RFC1918 IP space (IPv6-MW)

Seth Mattinen sethm at
Thu Feb 5 00:08:13 UTC 2009

Patrick W. Gilmore wrote:
> On Feb 4, 2009, at 6:56 PM, Scott Howard wrote:
>> On Mon, Feb 2, 2009 at 9:35 PM, Patrick W. Gilmore
>> <patrick at>wrote:
>>> Except the RIRs won't give you another /48 when you have only used one
>>> trillion IP addresses.
>> Of course they will!  A /48 is only the equivalent of 65536 "networks"
>> (each
>> network being a /64).  Presuming that ISPs allocate /64 networks to each
>> connected subscriber, then a /48 is only 65k subscribers, or say around a
>> maximum of 200k IP addresses in use at any one time (presuming no NAT
>> and an
>> average of 3-4 IP-based devices per subscriber)
>> IPv4-style utilization ratios do make some sense under IPv6, but not
>> at the
>> address level - only at the network level.
> First, it was (mostly) a joke.
> Second, where did you get 4 users per /64?  Are you planning to hand
> each cable modem a /64?

That was the generally accepted subnet practice last time I had a
discussion about it on the ipv6-ops list. I'm not an ISP, but I have a
/48 and each subnet is a /64. Some devices will refuse to work if you
subnet smaller than a /64. (Yes, poorly designed, etc.)


More information about the NANOG mailing list