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

Daniel Stutzbach daniel at stutzbachenterprises.com
Thu Sep 17 00:31:27 CEST 2009

On Wed, Sep 16, 2009 at 4:59 PM, Greg Ewing <greg.ewing at canterbury.ac.nz>wrote:

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

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 :) ).

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

In a router or operating system kernel, an IPv4 address or netmask is stored in a 32-bit unsigned integer. Host addresses, network addresses, and netmasks satisfy the following properties, where & is the bitwise-AND operation:

network address & netmask == network address host address & netmask == network address of that host

A gateway is just a host that will forward packets. A gateway address has the same properties as a host address and isn't the same as the zero address.

Every host has a routing table that contains a list of (network address, gateway, netmask) tuples. To see the list, just run "/sbin/route -n" on any Linux system (and most other Unixes; root is not typically required) or "route print" on a Windows system.

To route a packet, the operating system essentially performs the following algorithm:

def find_gateway_for_destination(destination_address): for (network_address, gateway, netmask) in list_of_routing_table_entires: if network_address == destination_address & netmask: return gateway raise DestinationUnreachable

Furthermore, an IPNetwork object

needs to be able to represent a network address whose address part contains bits that aren't in the mask.

Network addresses never contain bits that aren't in the mask (due to the rule: "network address & netmask == network address").

-- Daniel Stutzbach, Ph.D. President, Stutzbach Enterprises, LLC <http://stutzbachenterprises.com> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20090916/c15f752a/attachment.htm>

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