It's Ars Tech's turn to bang the IPv4 exhaustion drum

Crist Clark Crist.Clark at
Wed Aug 20 14:33:25 CDT 2008

>>> On 8/20/2008 at 11:57 AM, Iljitsch van Beijnum <iljitsch at>
> On 20 aug 2008, at 20:34, Crist Clark wrote:
>> On a "true" P-to-P link, there is no netmask, no? A netmask is a
>> concept that applies to broadcast media, like Ethernet. Even if
>> you only have two hosts on an Ethernet link, it's not really
>> P-to-P in the strict sense.
> An interface needs a prefix length (subnet mask for those of us stuck
> in the '90s) so the system knows which addresses are directly  
> connected through the interface in question. Whether the link is
> to-point, broadcast or NBMA doesn't matter for that purpose.

No, that's my point. On a true point-to-point link, there is
only one other address on the link. That's what point-to-point
means. And no, that does not really mean there is an implied /32
(for IPv4) or /128 (for IPv6) on the link since that would tell
the system its the only address on the link.

For example, on the IPv4 ends gif(4) tunnel in my previous message,

gif0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> metric 0 mtu 1280
        tunnel inet -->
        inet6 fe80::200:24ff:feca:91b4%gif0 prefixlen 64 scopeid 0x7 
        inet6 2001:470:1f04:2fc::2 --> 2001:470:1f04:2fc::1 prefixlen

A netmask doesn't make sense. They're not on the same LAN since
there is no LAN on a point-to-point tunnel. (The most specific
mask those two share is 0x8000000.)

As for the IPv6 portion, the two endpoints happen to be adjacent,
they look like they are "on the same network," but there is no reason
that has to be in the general case just like the IPv4 case. There is
no LAN. It's point-to-point. It could be,

   2001:470:8045:0:2b0:d0ff:fe2c:982d --> 2001:470:1f04:2fc::1

(That only happen share the /16 belonging to the ISP, not even
on the same /64) and everything would be fine.

Right? Or is something different about IPv6?

I'm wondering if all of this confusion is about people calling
an Ethernet, or other broadcast media, link with only two interfaces
on it point-to-point. It's not.

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