[Python-Dev] PEP 3144 review.

[Andrew McNamara]

>> Some people have claimed that the gateway address of a
>> network isn't necessarily the zero address in that network.

It almost never is - conventions vary, but it is often the network address
plus one, or the broadcast address minus one.

>I'll go further: I don't think it's even legal for the gateway address to be
>the zero address of the network (and I used to program the embedded software
>in routers for a living :) ).

I don't think the RFCs forbid the zero address being used, and
"enlightened" network stacks allow it (typically routers) to achieve
better utilisation of the limited IPv4 address space (for a /24 or larger,
wasting one address out of 255 isn't too bad, but it is now typical to
use much smaller nets - right down to /30).

>> If that's true, then you *can't* calculate the network
>> address from a host address and a netmask -- there isn't
>> enough information.

You can always calculate the network address from the IP address plus
mask - the network address is simply the bits that are not masked. 

In the olden days, the mask was spelled out in octets (eg
255.255.255.0). But we've moved to a more compact and logical notation
where the number of leading significant bits is specified (eg /24).

-- 
Andrew McNamara, Senior Developer, Object Craft
http://www.object-craft.com.au/