Make ipv4_to_ipv6 output "human-readable" addrs

The requirements keep changing back and forth, but apparently, it
was still important that the resulting IPv6 address was somewhat
human-readably similar to the original IPv4 address.

Also, it isn't always necessary to keep the full IPv4 address inside
the IPv6 address. Mostly, the host address from the IPv4 subnetwork
should be readable as the same on the corresponding IPV6 subnetwork,
so an option to keep only a number of IPv4 octets in the result was
added.

src/cnaas_nms/tools/jinja_filters.py

@@ -62,29 +62,37 @@ def isofy_ipv4(ip_string, prefix=''):
 
 
 def ipv4_to_ipv6(
-    v4_address: Union[str, ipaddress.IPv4Interface], v6_network: Union[str, ipaddress.IPv6Network]
+    v4_address: Union[str, ipaddress.IPv4Interface],
+    v6_network: Union[str, ipaddress.IPv6Network],
+    keep_octets: int = 1,
 ):
-    """Transforms an IPv4 address to an IPv6 interface address. This will combine an arbitrary
-    IPv6 network address with the 32 address bytes of an IPv4 address into a valid IPv6 address
-    + prefix length notation - the equivalent of dotted quad compatible notation.
+    """Transforms an IPv4 address intoto an IPv6 interface address that should be more-or-less
+    identifiable as the same device on a different protocol family.
 
-    E.g.:
-    >>> ipv6 = ipv4_to_ipv6("127.0.0.1", "2001:700:dead:babe::/64")
-    >>> ipv6
-    IPv6Interface('2001:700:dead:babe::7f00:1/64')
-    >>> ipv6 == ipaddress.IPv6Interface('2001:700:dead:babe::127.0.0.1/64')
-    True
+    A selected number of octets, right-to-left, of the IPv4 address will be converted to
+    hexadecimal notation that can be read as the decimal IPv4 address. This will then be used as
+    a host address on the provided IPv6 network.
+
+    Examples:
+    >>> ipv4_to_ipv6("10.1.0.42", "2001:700:dead:babe::/64")
+    IPv6Interface('2001:700:dead:babe::42/64')
+    >>> ipv4_to_ipv6("10.1.0.42", "2001:700:dead:babe::/64", keep_octets=4)
+    IPv6Interface('2001:700:dead:babe:10:1:0:42/64')
 
     Args:
-        v6_network: IPv6 network in prefix notation
         v4_address: IPv4 address
+        v6_network: IPv6 network in prefix notation
+        keep_octets: The number of octets to keep from the IPv4 address (from right-to-left)
     Returns:
         An IPv6Address object on the given network
     """
     if isinstance(v6_network, str):
         v6_network = ipaddress.IPv6Network(v6_network)
     if isinstance(v4_address, str):
         v4_address = ipaddress.IPv4Address(v4_address)
+    assert keep_octets > 0 <= 4
 
-    v6_address = v6_network[int(v4_address)]
+    octets = str(v4_address).split('.')[-keep_octets:]
+    v6ified = ipaddress.IPv6Address("::" + ":".join(octets))
+    v6_address = v6_network[int(v6ified)]
     return ipaddress.IPv6Interface(f"{v6_address}/{v6_network.prefixlen}")

src/cnaas_nms/tools/tests/test_jinja_filters.py

@@ -69,13 +69,13 @@ class IPv6ToIPv4Tests(unittest.TestCase):
     def test_should_convert_short_network_properly(self):
         self.assertEqual(
             ipv4_to_ipv6('10.0.0.1', '2001:700::/64'),
-            ipaddress.IPv6Interface('2001:700::10.0.0.1/64'),
+            ipaddress.IPv6Interface('2001:700::1/64'),
         )
 
     def test_should_convert_long_network_properly(self):
         self.assertEqual(
-            ipv4_to_ipv6('10.0.0.1', '2001:700:dead:c0de:babe::/80'),
-            ipaddress.IPv6Interface('2001:700:dead:c0de:babe::10.0.0.1/80'),
+            ipv4_to_ipv6('10.0.0.1', '2001:700:dead:c0de::/64', keep_octets=4),
+            ipaddress.IPv6Interface('2001:700:dead:c0de:10:0:0:1/64'),
         )
 
     def test_should_raise_on_invalid_network(self):