ISP customer assignments

[cordmacleod at gmail.com (Cord MacLeod)]

On Oct 13, 2009, at 4:26 PM, Chris Adams wrote:

> Once upon a time, Michael Dillon <wavetossed at googlemail.com> said:
>>> How many addresses do you like on point-to-point circuits?
>>
>> That will become one of those great interview questions, because  
>> anyone who says
>> something like "a /127" or "a /64" will be someone that you probably
>> don't want to hire.
>>
>> The right answer is to explain that there are some issues surrounding
>> the choice of
>> addressing on point-to-point circuits and there has even been an RFC
>> published discussing
>> these issues, RFC 3627 <http://www.ietf.org/rfc/rfc3627.txt>
>
> Still learning here, so please go easy...
>
> I read the above, and I see section 4 item 3 says:
>
>   The author feels that if /64 cannot be used, /112, reserving the  
> last
>   16 bits for node identifiers, has probably the least amount of
>   drawbacks (also see section 3).
>
> I guess I'm missing something; what in section 3 is this referring to?
> I can understand /64 or /126 (or maybe /124 if you were going to
> delegate reverse DNS?), but why /112 and "16 bits for node  
> identifiers"
> on a point-to-point link?

I'm actually completely unclear why people would use anything but a / 
126 in 90% or more of cases.  For all intensive purposes a /126  
translates to a /30 in IPv4.  Do people assign /24's to their point to  
point links today with IPv4?  What's the point of a /64 on a point to  
point link?  I'm not clear why people would intentionally be so  
frivolous with their IP space simply for the sake of "because I can."