Fixes #6350: Include first & last IP addresses when allocating available IPv6 addresses via the REST API

docs/release-notes/version-2.11.md

@@ -18,6 +18,7 @@
 * [#6321](https://github.com/netbox-community/netbox/issues/6321) - Restore "add an IP" button under prefix IPs view
 * [#6333](https://github.com/netbox-community/netbox/issues/6333) - Fix filtering of circuit terminations by primary key
 * [#6339](https://github.com/netbox-community/netbox/issues/6339) - Improve ordering of interfaces when viewing virtual chassis master
+* [#6350](https://github.com/netbox-community/netbox/issues/6350) - Include first & last IP addresses when allocating available IPv6 addresses via the REST API
 * [#6355](https://github.com/netbox-community/netbox/issues/6355) - Fix caching error when swapping A/Z circuit terminations
 * [#6357](https://github.com/netbox-community/netbox/issues/6357) - Fix ProviderNetwork nested API serializer
 * [#6363](https://github.com/netbox-community/netbox/issues/6363) - Correct pre-population of cluster group when creating a cluster

netbox/ipam/models/ip.py

@@ -426,19 +426,11 @@ class Prefix(PrimaryModel):
         child_ips = netaddr.IPSet([ip.address.ip for ip in self.get_child_ips()])
         available_ips = prefix - child_ips
 
-        # All IP addresses within a pool are considered usable
-        if self.is_pool:
+        # IPv6, pool, or IPv4 /31 sets are fully usable
+        if self.family == 6 or self.is_pool or self.prefix.prefixlen == 31:
             return available_ips
 
-        # All IP addresses within a point-to-point prefix (IPv4 /31 or IPv6 /127) are considered usable
-        if (
-            self.prefix.version == 4 and self.prefix.prefixlen == 31  # RFC 3021
-        ) or (
-            self.prefix.version == 6 and self.prefix.prefixlen == 127  # RFC 6164
-        ):
-            return available_ips
-
-        # Omit first and last IP address from the available set
+        # For "normal" IPv4 prefixes, omit first and last addresses
         available_ips -= netaddr.IPSet([
             netaddr.IPAddress(self.prefix.first),
             netaddr.IPAddress(self.prefix.last),