[Python-Dev] PEP 3144 review. (original) (raw)

Andrew McNamara andrewm at object-craft.com.au
Fri Sep 18 06:26:19 CEST 2009

On Fri, 18 Sep 2009 11:04:46 am Andrew McNamara wrote:

>To a non-specialist, "the network address" is ambiguous. There are > many addresses in a network, and none of them are the entire > network. It's like saying, given a list [2, 4, 8, 12], what's "the > list item"?

A "network address" is an IP address and mask, but I understand your confusion - we're mixing terminology from disperate domains. In my postings, I have tried to refer to Network (a containter) and Address (an item). So to clarify, how many different things which need to be handled are there? Items: 1 IP address -- a 32 bit (IPv4) or 128 bit (IPv6) number

Yes.

2 Netmask -- a bit mask of the form 111..100..0

I don't think there's much to be gained by exposing a Netmask object, although other objects might have a .netmask property returning an IPAddress instance. Where we expose a netmask, it should be as an Address instance (or maybe a subclass with additional restrictions).

3 Network address -- the lowest address in a network, and equal to (defined by?) the bitwise-AND of any address in the network with the network's netmask

This idea of a "network address" being simply an IP address is in error - a network address was always an address and a mask, however in the days prior to CIDR, the mask was implicitly specified by the class of the network.

4 Host address -- the part of the IP address that is not masked by the netmask

Well, yes, but I don't think we need an entity representing that.

5 Broadcast address -- the highest address in a IPv4 network

Yes, but again, we don't need an entity - as with the netmask, when exposed, it should just be an Address instance (or subclass thereof).

Containers: 6 Network -- a range of IP address

Yes, although not an arbitrary or discontinuous range of address.

Really, I think we just need two entities (per protocol):

Address (& maybe AddressWithMask)

Network

So, you want to represent an interface on your host:

if_addr = IPv4Address('10.0.0.1/24')

from this, you could get:

if_addr.address IPv4Address('10.0.0.1') if_addr.netmask IPv4Address('255.255.255.0') if_addr.broadcast IPv4Address('10.0.0.255') if_addr.network IPV4Network('10.0.0.0/24')

you might also have an address for the default gateway:

router_addr = IPv4Address('10.0.0.254/24') router_addr in if_addr.network True

or:

router_addr = IPv4Address('10.0.0.254') router_addr in if_addr.network True

Or maybe you've subneted your LAN:

IPV4Network('10.0.0.0/24') in IPv4Network('10.0.0.0/8') True IPV4Network('10.0.1.0/24') in IPv4Network('10.0.0.0/8') True

but:

IPV4Network('10.0.0.0/8') in IPv4Network('10.0.0.0/24') False

This suggests the natural behaviour if the Address mask doesn't fit in the network:

IPv4Address('10.0.0.254/8') in IPv4Network('10.0.0.0/24') False

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

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