New: Improve IPAddress.IsLocal method

Closes #2032

(cherry picked from commit fd98a179ab6fed8037c99344b34593aac24a0ac0)
Co-Authored-By: ta264 <ta264@users.noreply.github.com>

src/NzbDrone.Common/Extensions/IpAddressExtensions.cs

@@ -1,4 +1,5 @@
 using System.Net;
+using System.Net.Sockets;
 
 namespace NzbDrone.Common.Extensions
 {
@@ -6,24 +7,50 @@ namespace NzbDrone.Common.Extensions
     {
         public static bool IsLocalAddress(this IPAddress ipAddress)
         {
-            if (ipAddress.ToString() == "::1")
+            // Map back to IPv4 if mapped to IPv6, for example "::ffff:1.2.3.4" to "1.2.3.4".
+            if (ipAddress.IsIPv4MappedToIPv6)
+            {
+                ipAddress = ipAddress.MapToIPv4();
+            }
+
+            // Checks loopback ranges for both IPv4 and IPv6.
+            if (IPAddress.IsLoopback(ipAddress))
             {
                 return true;
             }
 
-            byte[] bytes = ipAddress.GetAddressBytes();
-            switch (bytes[0])
+            // IPv4
+            if (ipAddress.AddressFamily == AddressFamily.InterNetwork)
             {
-                case 10:
-                case 127:
-                    return true;
-                case 172:
-                    return bytes[1] < 32 && bytes[1] >= 16;
-                case 192:
-                    return bytes[1] == 168;
-                default:
-                    return false;
+                return IsLocalIPv4(ipAddress.GetAddressBytes());
             }
+
+            // IPv6
+            if (ipAddress.AddressFamily == AddressFamily.InterNetworkV6)
+            {
+                return ipAddress.IsIPv6LinkLocal ||
+                        ipAddress.IsIPv6UniqueLocal ||
+                        ipAddress.IsIPv6SiteLocal;
+            }
+
+            return false;
+        }
+
+        private static bool IsLocalIPv4(byte[] ipv4Bytes)
+        {
+            // Link local (no IP assigned by DHCP): 169.254.0.0 to 169.254.255.255 (169.254.0.0/16)
+            bool IsLinkLocal() => ipv4Bytes[0] == 169 && ipv4Bytes[1] == 254;
+
+            // Class A private range: 10.0.0.0 – 10.255.255.255 (10.0.0.0/8)
+            bool IsClassA() => ipv4Bytes[0] == 10;
+
+            // Class B private range: 172.16.0.0 – 172.31.255.255 (172.16.0.0/12)
+            bool IsClassB() => ipv4Bytes[0] == 172 && ipv4Bytes[1] >= 16 && ipv4Bytes[1] <= 31;
+
+            // Class C private range: 192.168.0.0 – 192.168.255.255 (192.168.0.0/16)
+            bool IsClassC() => ipv4Bytes[0] == 192 && ipv4Bytes[1] == 168;
+
+            return IsLinkLocal() || IsClassA() || IsClassC() || IsClassB();
         }
     }
 }