[issue22876] ip_interface can't be broadcast or number net

[Nick Coghlan]

Nick Coghlan added the comment:

The Interface classes are actually designed to cover any association of an IP 
address with a specific network, not just host interface addresses. We knew it 
was a slight misnomer when we chose it, but network and broadcast addresses 
weren't considered special enough to be worth separating out (beyond the 
distinction present in iterating over the whole network range vs just the host 
addresses).

It's relatively straightforward to write a helper function that only allows the 
creation of host interfaces:

def host_interface(address):
ifaddr = ip_interface(address)
if ifaddr.ip == ifaddr.network.network_address:
raise ValueError("'{}' is a network address".format(ifaddr)
if ifaddr.ip == ifaddr.network.broadcast_address:
raise ValueError("'{}' is a broadcast address".format(ifaddr)
return ifaddr

I'm not sure if it's worthwhile to add such a helper function to the module 
itself.

One argument in favour of adding it is that it may help to emphasise that the 
normal ip_interface factory function and the *Interface class constructors are 
*not* restricted purely to host interfaces, and thus allow network and 
broadcast addresses to be associated with their corresponding network 
definition.

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[issue22876] ip_interface can't be broadcast or number net

[Вячеслав]

Вячеслав added the comment:

Probably worth noting that network is unnumbered in ip_network and ip_interface 
functions. Based on this flag to decide whether there is a possibility to use 
the network address and broadcast address in the network
What do you think about this?

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[issue22876] ip_interface can't be broadcast or number net

[Вячеслав]

Вячеслав added the comment:

These addresses are used by each interface in the network and they can not be 
the address of the interface, of course have the technology ip-unnumbered. But 
it's more a special case.

but I was confused behavior:

>>> set(ip_network(u'192.168.1.0/29'))
set([IPv4Address(u'192.168.1.5'), IPv4Address(u'192.168.1.6'), 
IPv4Address(u'192.168.1.7'), IPv4Address(u'192.168.1.2'), 
IPv4Address(u'192.168.1.3'), IPv4Address(u'192.168.1.4'), 
IPv4Address(u'192.168.1.0'), IPv4Address(u'192.168.1.1')])


>>> for i in ip_network(u'192.168.1.0/29').hosts():
... print i
...
192.168.1.1
192.168.1.2
192.168.1.3
192.168.1.4
192.168.1.5
192.168.1.6

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[issue22876] ip_interface can't be broadcast or number net

[R. David Murray]

R. David Murray added the comment:

Well, it can be the network, even though it isn't typically (and some devices 
don't support it...I'm pretty sure I remember doing it on a Cisco, though I 
wouldn't swear to it without testing :).  Same is true for broadcast, though 
that would be *really* questionable and I doubt many devices support it.  (At 
least, I couldn't find any RFC that says those two addresses are actually 
reserved in all contexts).  

I know of two situations in which it is specifically not true: the simplest 
(which could be special cased) is a two ip (point to point) subnet.  The other 
is a "routed" subnet: a subnet where the subnet is routed to a router/firewall, 
and the router/firewall NATs those addresses to some internal addresses.  
Conceptually, every IP in the subnet can be an interface IP.

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nosy: +r.david.murray
versions: +Python 3.4, Python 3.5 -Python 2.7

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[issue22876] ip_interface can't be broadcast or number net

[Вячеслав]

New submission from Вячеслав:

ip_interface can't be network or broadcast address. Probably should throw an 
exception in such cases

>>> ip_interface(u'192.168.1.0/25')
IPv4Interface(u'192.168.1.0/25')

>>> ip_interface(u'192.168.1.127/25')
IPv4Interface(u'192.168.1.127/25')

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components: Library (Lib)
messages: 231194
nosy: m01, ncoghlan, pmoody, python-dev, Вячеслав
priority: normal
severity: normal
status: open
title: ip_interface can't be broadcast or number net
type: behavior
versions: Python 2.7

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