Foundations of Python Network Programming 978-1-4302-3004-5

CHAPTER 4 ■ SOCKET NAMES AND DNS
But we should immediately step back, and note that it is only because of our specific choices for the
first three coordinates that our socket names have had two components, hostname and port! If you
instead had chosen AppleTalk or ATM or Bluetooth for your address family, then some other data
structure might have been required of you instead of a tuple with a string and an integer inside. So the
whole set of coordinates, which in this section we have talked about as five coordinates, is really three
fixed coordinates needed to create the socket, followed by however many more coordinates your
particular address family requires you to use in order to make a network connection.
IPv6
And having explained all of that, it turns out that this book actually does need to introduce one
additional address family, beyond the AF_INET we have used so far: the address family for IPv6, named
AF_INET6, which is the way forward into a future where the world does not, in fact, run out of IP
addresses.
Once the old ARPANET really started taking off, its choice of 32-bit address names—which made so
much sense back when computer memory chips were measured by the kilobyte—became a clear and
worrying limitation. With only about four billion possible addresses available, even assuming that we
could use the address space that fully, that makes fewer than one IP address for every person on the
earth—which means real trouble once everyone has both a computer and an iPhone!
Even though only a few percent of the computers on the Internet today are actually using IPv6 to
communicate with the global network through their Internet service providers (where “today” is
September 2010), the steps necessary to make your Python programs compatible with IPv6 are simple
enough that you should go ahead and try writing code that prepares us all for the future.
In Python you can test directly for whether the underlying platform supports IPv6 by checking the
has_ipv6 Boolean attribute inside the socket module:
>>> import socket
>>> socket.has_ipv6
True
But note that this does not tell you whether an actual IPv6 interface is up and configured and can
currently be used to send packets anywhere; it is purely an assertion about whether IPv6 support has
been compiled into the operating system, not about whether it is in use!
The differences that IPv6 will make for your Python code might sound quite daunting, if listed one
right after the other:
• Your sockets have to be prepared to have the family AF_INET6 if you are called
upon to operate on an IPv6 network.
• No longer do socket names consist of just two pieces, an address and a port
number; instead, they can also involve additional coordinates that provide “flow”
information and a “scope” identifier.
• The pretty IPv4 octets like 18.9.22.69 that you might already be reading from
configuration files or from your command-line arguments will now sometimes be
replaced by IPv6 host addresses instead, which you might not even have good
regular expressions for yet. They have lots of colons, they can involve hexadecimal
numbers, and in general they look quite ugly.
The benefits of the IPv6 transition are not only that it will make an astronomically large number of
addresses available, but also that the protocol has more complete support for things like link-level
security than do most implementations of IPv4.
But the changes just listed can sound like a lot of trouble if you have been in the habit of writing
clunky, old-fashioned code that puts IP addresses and hostnames through regular expressions of your
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