Fixes #2757: Always treat first/last IPs within a /31 or /127 as usable

CHANGELOG.md

@@ -12,6 +12,7 @@ v2.5.3 (FUTURE)
 ## Bug Fixes
 
 * [#2742](https://github.com/digitalocean/netbox/issues/2742) - Preserve cluster assignment when editing a device
+* [#2757](https://github.com/digitalocean/netbox/issues/2757) - Always treat first/last IPs within a /31 or /127 as usable
 * [#2762](https://github.com/digitalocean/netbox/issues/2762) - Add missing DCIM field values to API `_choices` endpoint
 
 

netbox/ipam/models.py

@@ -438,12 +438,23 @@ class Prefix(ChangeLoggedModel, CustomFieldModel):
         child_ips = netaddr.IPSet([ip.address.ip for ip in self.get_child_ips()])
         available_ips = prefix - child_ips
 
-        # Remove unusable IPs from non-pool prefixes
-        if not self.is_pool:
-            available_ips -= netaddr.IPSet([
-                netaddr.IPAddress(self.prefix.first),
-                netaddr.IPAddress(self.prefix.last),
-            ])
+        # All IP addresses within a pool are considered usable
+        if self.is_pool:
+            return available_ips
+
+        # All IP addresses within a point-to-point prefix (IPv4 /31 or IPv6 /127) are considered usable
+        if (
+            self.family == 4 and self.prefix.prefixlen == 31  # RFC 3021
+        ) or (
+            self.family == 6 and self.prefix.prefixlen == 127  # RFC 6164
+        ):
+            return available_ips
+
+        # Omit first and last IP address from the available set
+        available_ips -= netaddr.IPSet([
+            netaddr.IPAddress(self.prefix.first),
+            netaddr.IPAddress(self.prefix.last),
+        ])
 
         return available_ips