Handle RFC 6164 IPv6 addressespick-191

Like RFC 3021, IPv6 defines point-to-point subnets that must
be handled separately, i.e. don't reserve first IP address.

This patch aims to implement this, while refactoring code in
iter_host function to reduce code duplication.

Tests for this feature also added to ensure there is no regression.

Signed-off-by: Damien Claisse <d.claisse@criteo.com>

3 files changed, 43 insertions, 28 deletions

diff --git a/netaddr/ip/__init__.py b/netaddr/ip/__init__.py
index b61717b..edcbaf1 100644
--- a/netaddr/ip/__init__.py
+++ b/netaddr/ip/__init__.py
@@ -1004,7 +1004,7 @@ class IPNetwork(BaseIP, IPListMixin):
     @property
     def broadcast(self):
         """The broadcast address of this `IPNetwork` object"""
-        if self._module.version == 4 and (self._module.width - self._prefixlen) <= 1:
+        if (self._module.width - self._prefixlen) <= 1:
             return None
         else:
             return IPAddress(self._value | self._hostmask_int, self._module.version)
@@ -1306,36 +1306,38 @@ class IPNetwork(BaseIP, IPListMixin):
         A generator that provides all the IP addresses that can be assigned
         to hosts within the range of this IP object's subnet.
 
-        - for IPv4, the network and broadcast addresses are always excluded. \
-          for subnets that contains less than 4 IP addresses /31 and /32 \
-          report in a manner per RFC 3021
+        - for IPv4, the network and broadcast addresses are excluded, excepted \
+          when using /31 or /32 subnets as per RFC 3021.
 
-        - for IPv6, only the unspecified address '::' or Subnet-Router anycast \
-          address (first address in the network) is excluded.
+        - for IPv6, only Subnet-Router anycast address (first address in the \
+          network) is excluded as per RFC 4291 section 2.6.1, excepted when using \
+          /127 or /128 subnets as per RFC 6164.
 
         :return: an IPAddress iterator
         """
         it_hosts = iter([])
 
+        # Common logic, first IP is always reserved.
+        first_usable_address = self.first + 1
         if self._module.version == 4:
-            #   IPv4 logic.
-            if self.size >= 4:
-                it_hosts = iter_iprange(
-                        IPAddress(self.first + 1, self._module.version),
-                        IPAddress(self.last - 1, self._module.version))
-            else:
-                it_hosts = iter_iprange(
-                        IPAddress(self.first, self._module.version),
-                        IPAddress(self.last, self._module.version))
+            # IPv4 logic, last address is reserved for broadcast.
+            last_usable_address = self.last - 1
+        else:
+            # IPv6 logic, no broadcast address reserved.
+            last_usable_address = self.last
+
+        # If subnet has a size of less than 4, then it is a /31, /32, /127 or /128.
+        # Handle them as per RFC 3021 (IPv4) or RFC 6164 (IPv6), and don't reserve
+        # first or last IP address.
+        if self.size >= 4:
+            it_hosts = iter_iprange(
+                    IPAddress(first_usable_address, self._module.version),
+                    IPAddress(last_usable_address, self._module.version))
         else:
-            #   IPv6 logic.
-            # RFC 4291 section 2.6.1 says that the first IP in the network is
-            # the Subnet-Router anycast address. This address cannot be
-            # assigned to a host, so use self.first+1.
-            if self.size >= 2:
-                it_hosts = iter_iprange(
-                    IPAddress(self.first + 1, self._module.version),
+            it_hosts = iter_iprange(
+                    IPAddress(self.first, self._module.version),
                     IPAddress(self.last, self._module.version))
+
         return it_hosts
 
     def __str__(self):
diff --git a/netaddr/tests/ip/test_ip_v4.py b/netaddr/tests/ip/test_ip_v4.py
index f6d774e..46e22b3 100644
--- a/netaddr/tests/ip/test_ip_v4.py
+++ b/netaddr/tests/ip/test_ip_v4.py
@@ -260,8 +260,6 @@ def test_iterhosts_v4():
         IPAddress('192.168.0.1'),
     ]
 
-    assert list(IPNetwork("1234::/128")) == [IPAddress('1234::')]
-    assert list(IPNetwork("1234::/128").iter_hosts()) == []
     assert list(IPNetwork("192.168.0.0/31").iter_hosts()) == [IPAddress('192.168.0.0'),IPAddress('192.168.0.1')]
     assert list(IPNetwork("192.168.0.0/32").iter_hosts()) == [IPAddress('192.168.0.0')]
 
@@ -509,10 +507,6 @@ def test_rfc3021_subnets():
     assert IPNetwork('192.0.2.0/32').broadcast is None
     assert list(IPNetwork('192.0.2.0/32').iter_hosts()) == [IPAddress('192.0.2.0')]
 
-    # IPv6 must not be affected
-    assert IPNetwork('abcd::/127').broadcast is not None
-    assert IPNetwork('abcd::/128').broadcast is not None
-
 
 def test_ipnetwork_change_prefixlen():
     ip = IPNetwork('192.168.0.0/16')
diff --git a/netaddr/tests/ip/test_ip_v6.py b/netaddr/tests/ip/test_ip_v6.py
index aa9c5c1..5b5486a 100644
--- a/netaddr/tests/ip/test_ip_v6.py
+++ b/netaddr/tests/ip/test_ip_v6.py
@@ -141,3 +141,22 @@ def test_ipv6_unicast_address_allocation_info():
     assert ip.info.IPv6_unicast[0].description == 'LACNIC'
     assert ip.info.IPv6_unicast[0].whois == 'whois.lacnic.net'
     assert ip.info.IPv6_unicast[0].status == 'ALLOCATED'
+
+def test_rfc6164_subnets():
+    # Tests for /127 subnet
+    assert list(IPNetwork('1234::/127')) == [
+        IPAddress('1234::'),
+        IPAddress('1234::1'),
+    ]
+    assert list(IPNetwork('1234::/127').iter_hosts()) == [
+        IPAddress('1234::'),
+        IPAddress('1234::1'),
+    ]
+    assert IPNetwork('1234::/127').network == IPAddress('1234::')
+    assert IPNetwork('1234::').broadcast is None
+
+    # Tests for /128 subnet
+    assert IPNetwork("1234::/128").network == IPAddress('1234::')
+    assert IPNetwork("1234::/128").broadcast is None
+    assert list(IPNetwork("1234::/128")) == [IPAddress('1234::')]
+    assert list(IPNetwork("1234::/128").iter_hosts()) == [IPAddress('1234::')]