v6 & DSL / Cable modems [was: Private use of non-RFC1918 IP space (IPv6-MW)] (IPv6-MW)

Nathan Ward nanog at daork.net
Thu Feb 5 01:56:30 UTC 2009

On 5/02/2009, at 2:35 PM, Scott Howard wrote:

> What happens when a customer wants to run multiple networks is  
> something I
> haven't seen answered yet - with NAT it's easy, but as I said, NAT is
> apparently evil...

You give them more than a /64.

RFC4291 says that it should be a /48, but people seem to be keen on / 
56s now. /60s are even ok.
They key here is that is is divisible by 4, which leaves full hex  
digits for the customer to twiddle. Somewhere (free.fr?) are doing / 
61, which is a bit tough for people that aren't so technical.

Here in NZ, users typically purchase their own ADSL CPE, and that runs  
PPPoATM over ADSL, and does IPv4 NAT and so on. What is also common,  
is people buy a "wireless router" and plug it in to the back of their  
ADSL router. They now have two layers of NAT between wireless hosts  
and the Internet.

I looked at a packet trace of outgoing packets from an ISP - 17% of  
outgoing packets were from behind double NAT like this (TTL was 62 or  
126, as opposed to 63 or 127).

For this topology to work in IPv6, multiple levels of PD are required,  
or users can no longer do this sort of plug-and-pray networking. Fun  

Personally, I think we should have PD forwarding - ie. a router can  
forward PD requests from routers behind it up to the ISP, and the ISP  
can dish out another /64. It means there are more routes in that  
particular router at the ISP, but it means you don't have to worry  
about how much address space to give to each customer - if they need  
more they ask for it automatically.

Nathan Ward

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