address: is the primary address and it is always set.
 - netmask: and 'broadcast' and 'ptp' may be None.
 - ptp: stands for "point to point" and references the
        destination address on a point to point interface
        (typically a VPN).
 - broadcast: and *ptp* are mutually exclusive.

Note: you can have more than one address of the same family
associated with each interface.
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