Fixes #427: Prevent error when duplicate IPs are present in a prefix's IP list

netbox/ipam/views.py

@@ -66,10 +66,10 @@ def add_available_ipaddresses(prefix, ipaddress_list):
     # Iterate through existing IPs and annotate free ranges
     for ip in ipaddress_list:
         if prev_ip:
-            skipped_count = int(ip.address.ip - prev_ip.address.ip - 1)
-            if skipped_count:
+            diff = int(ip.address.ip - prev_ip.address.ip)
+            if diff > 1:
                 first_skipped = '{}/{}'.format(prev_ip.address.ip + 1, prefix.prefixlen)
-                output.append((skipped_count, first_skipped))
+                output.append((diff - 1, first_skipped))
         output.append(ip)
         prev_ip = ip