[Python-Dev] PEP 3144 review.

[Sebastian Rittau]

On Tue, Sep 15, 2009 at 01:16:06PM -0400, Scott Dial wrote:

> I have to concur with the opinions above. I was very confused by the
> following error:
> 
> >>> addr = ipaddr.IPAddress("10.1.2.3/255.255.240.0")
> ...
> ipaddr.IPAddressIPValidationError: '98.223.189.24/255.255.240.0' is not
> a valid address (hint, it's probably a network)
> 
> Because, it *is* a address of a host on a network.

To me, 10.1.2.3/255.255.240.0 is not a host address, but specifies a
network. I.e., 10.1.2.3/255.255.240.0 == 10.1.0.0/255.255.240.0 ==
10.1.35.200/20.

> >>> net = ipaddr.IPNetwork("10.1.2.3/255.255.240.0")
> 
> But then, I was dumbfounded as to how I could get the gateway IP from
> this IPNetwork object.

Well, you can't. There is no way to determine a gateway, without querying
the network topology. This is clearly outside the scope of this module.
The only two known host addresses of a network are the network address
(10.1.0.0 in the example you gave) and the broadcast address (10.1.15.255).

> It took me a while to figure out that you can
> iterate over IPNetwork instances:
> 
> >>> gateway = net[1]
> 
> I was then confused, because:
> 
> >>> print(type(gateway))
> <class 'ipaddr.IPv4Address'>
> 
> Which sorta blew my mind.. I fully expected to receive an IPNetwork back
> from that operation. It is unclear to me why the network information
> gets chucked by that operation.

This makes perfect sense to me. An IP network consists of a list of IP
addresses. Returning /32 networks seems kind of pointless to me.

> I foresee having to work around that in
> real applications by doing something obnoxious like:
> 
> >>> actual_gateway = ipaddr.IPNetwork("%s/%s" % (gateway, addr.netmask))

But a gateway is not an IP address plus hostmask. A gateway is just a single
IP address. What is the use of adding a hostmask to the gateway IP address
or some other IP address inside the network?

 - Sebastian