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

Sebastian Rittau srittau at jroger.in-berlin.de
Tue Sep 15 21:35:13 CEST 2009

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:

>>> actualgateway = 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?

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