On Mon, Jun 1, 2009 at 8:05 PM, R. David Murray <report@bugs.python.org> wrote:

> In pre-CIDR days, assuming a prefixlen of 24 for a 192.168.x.x address
> made sense.  Nowadays it is better not to make that assumption.  So I
> find ipaddr's default of 32 to be "safer" than using a class based default.

Sorry, but why should you determine what is better for my application?

> The larger point, however, is that there _is_ a mask associated with the
> address in ifconfig.  There _must_ be one.  So that is not an example
> that shows that a separate address class is useful.

Again, you're wrong. The mask that you see in ifconfig is associated
with the network to which the interface is attached, not the interface
address. You also see a broadcast address in the ifconfig output, but
certainly you don't believe that the broadcast address is a property
of the interface address? No, of course not; it's a property of the
network to which the interface is attached -- just like the mask.

That's why we call it a "netmask".

> As for an example of when the equivalence is useful, it is useful every
> time I set up an access rule or route that applies to a single host.

Host routes are routes like all others -- they have a destination
address and a mask. That a host route has a prefix length of 32 does
not imply that the host route is equivalent to the host address.
You're confusing the two concepts.

> Most networking
> software that I've dealt with requires explicit netmasks (often with a
> shorthand to specify an ip/hostmask pair).

By "ip/hostmask" pair, you actually mean "ip/netmask" pair, and yes,
this is a convenient notation for expressing two distinct but related
concepts: an address, and a mask. Addresses do not have masks;
networks do. I am not sure how to be any more clear about that point,
yet you still seem not to understand.

> It is true that when a
> netmask isn't required it generally defaults to the classful netmask,
> but having such a default is becoming more rare with time, in my
> experience (because of CIDR).

I'm not advocating classful routing, I'm merely stating (emphatically
and without ambiguity) that addresses and networks are different:
networks have masks; addresses do not. The ipaddr library forces a
mask on me every time I specify an address. In my view, that is a
design flaw.

Clay