diff --git a/plist b/plist
index c2b76b18ba..b50e4b1507 100644
--- a/plist
+++ b/plist
@@ -1083,6 +1083,7 @@
 /usr/local/opnsense/mvc/tests/app/library/OPNsense/Firewall/FilterRuleTest/testDirection.conf
 /usr/local/opnsense/mvc/tests/app/library/OPNsense/Firewall/FilterRuleTest/testIcmp.conf
 /usr/local/opnsense/mvc/tests/app/library/OPNsense/Firewall/FilterRuleTest/testProtocol.conf
+/usr/local/opnsense/mvc/tests/app/library/OPNsense/Interface/IdassocTest.php
 /usr/local/opnsense/mvc/tests/app/models/OPNsense/ACL/AclConfig/config.xml
 /usr/local/opnsense/mvc/tests/app/models/OPNsense/ACL/AclTest.php
 /usr/local/opnsense/mvc/tests/app/models/OPNsense/Base/BaseModel/Migrations/M0_0_1.php
diff --git a/src/opnsense/mvc/app/library/OPNsense/Interface/Idassoc.php b/src/opnsense/mvc/app/library/OPNsense/Interface/Idassoc.php
index 482112b8e1..30756aae47 100644
--- a/src/opnsense/mvc/app/library/OPNsense/Interface/Idassoc.php
+++ b/src/opnsense/mvc/app/library/OPNsense/Interface/Idassoc.php
@@ -45,7 +45,7 @@ class Idassoc extends Autoconf
         $bytes = array_values(unpack('C*', inet_pton($address)));
 
         $source_prefix_len = (int)$source_prefix_len;
-        $prefix_id = hexdec((string)$prefix_id);
+        $prefix_id = (int)$prefix_id;
         $id_bits = 64 - $source_prefix_len;
 
         for ($i = 0; $i < $id_bits; $i++) {
@@ -69,7 +69,7 @@ class Idassoc extends Autoconf
     private static function calculateUsablePrefixLength($source_prefix_len, $track6_prefix_id, $track6_prefix_range = ''): int
     {
         $source_prefix_len = (int)$source_prefix_len;
-        $track6_prefix_id = hexdec((string)$track6_prefix_id);
+        $track6_prefix_id = (int)$track6_prefix_id;
         $track6_prefix_range = (string)$track6_prefix_range !== '' ? (int)$track6_prefix_range : 1;
 
         $associated_size = 1 << (64 - $source_prefix_len);
diff --git a/src/opnsense/mvc/tests/app/library/OPNsense/Interface/IdassocTest.php b/src/opnsense/mvc/tests/app/library/OPNsense/Interface/IdassocTest.php
new file mode 100644
index 0000000000..3de20e42ac
--- /dev/null
+++ b/src/opnsense/mvc/tests/app/library/OPNsense/Interface/IdassocTest.php
@@ -0,0 +1,154 @@
+<?php
+
+/*
+ * Copyright (C) 2026 Deciso B.V.
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice,
+ *    this list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES,
+ * INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY
+ * AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,
+ * OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+ * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+ * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+ * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+ * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ */
+
+namespace tests\OPNsense\Interface;
+
+use OPNsense\Interface\Idassoc;
+use PHPUnit\Framework\TestCase;
+
+/*
+ * Test value mapping:
+ *
+ * $track6_interface_prefix:
+ *   Runtime IPv6 prefix associated with the source interface selected by:
+ *   interfaces.<ifname>.track6-interface
+ *   Format: IPv6 CIDR prefix, for example "2001:db8:1234:ab00::/56"
+ *
+ * $track6_interface_prefix_len:
+ *   Prefix length extracted from $track6_interface_prefix.
+ *   Format: decimal integer, for example 56
+ *
+ * $track6_prefix_id:
+ *   Configuration value:
+ *   interfaces.<ifname>.track6-prefix-id
+ *   Format: decimal string, for example "0", "10", "16", "255"
+ *   The prefix ID is stored as decimal string in the config, even though the GUI overlays it as hex.
+ *   https://github.com/opnsense/core/blob/fb514217bae6525f36834ebd7279a02ad8626f0f/src/www/interfaces.php#L327
+ *
+ * $track6_prefix_range:
+ *   Configuration value:
+ *   interfaces.<ifname>.track6_prefix_range
+ *   Format: decimal string, interpreted as number of /64 slots,
+ *   for example "1", "2", "4", "16"
+ *
+ * $expected:
+ *   Calculated output value consumed by services:
+ *   prefix_on_link or prefix_allocated
+ *   Format: IPv6 CIDR prefix, for example "2001:db8:1234:ab10::/64"
+ */
+class IdassocTest extends TestCase
+{
+    private function callPrivateStatic(string $method, array $args = [])
+    {
+        $reflection = new \ReflectionClass(Idassoc::class);
+        $method = $reflection->getMethod($method);
+        $method->setAccessible(true);
+
+        return $method->invokeArgs(null, $args);
+    }
+
+    private function assertCalculatedPrefix(
+        string $track6_interface_prefix,
+        string $track6_prefix_id,
+        string $expected
+    ) {
+        $this->assertEquals(
+            $expected,
+            $this->callPrivateStatic('calculatePrefix', [$track6_interface_prefix, $track6_prefix_id]),
+            sprintf(
+                'track6_interface_prefix=%s track6_prefix_id=%s',
+                $track6_interface_prefix,
+                $track6_prefix_id
+            )
+        );
+    }
+
+    private function assertUsablePrefixLength(
+        int $track6_interface_prefix_len,
+        string $track6_prefix_id,
+        string $track6_prefix_range,
+        int $expected
+    ) {
+        $this->assertEquals(
+            $expected,
+            $this->callPrivateStatic(
+                'calculateUsablePrefixLength',
+                [$track6_interface_prefix_len, $track6_prefix_id, $track6_prefix_range]
+            ),
+            sprintf(
+                'track6_interface_prefix_len=%s track6_prefix_id=%s track6_prefix_range=%s',
+                $track6_interface_prefix_len,
+                $track6_prefix_id,
+                $track6_prefix_range
+            )
+        );
+    }
+
+    public function testPrefixId()
+    {
+        $track6_interface_prefix = '2001:db8:1234:ab00::/56';
+
+        $this->assertCalculatedPrefix($track6_interface_prefix, '0', '2001:db8:1234:ab00::/64');
+        $this->assertCalculatedPrefix($track6_interface_prefix, '1', '2001:db8:1234:ab01::/64');
+        $this->assertCalculatedPrefix($track6_interface_prefix, '10', '2001:db8:1234:ab0a::/64');
+        $this->assertCalculatedPrefix($track6_interface_prefix, '15', '2001:db8:1234:ab0f::/64');
+
+        // Configuration stores decimal 16; GUI may display this as hexadecimal "10".
+        $this->assertCalculatedPrefix($track6_interface_prefix, '16', '2001:db8:1234:ab10::/64');
+
+        $this->assertCalculatedPrefix($track6_interface_prefix, '255', '2001:db8:1234:abff::/64');
+    }
+
+    public function testDelegationSize()
+    {
+        // /52 leaves 12 bits for the subnet ID until /64.
+        $this->assertCalculatedPrefix('2001:db8:1234:a000::/52', '16', '2001:db8:1234:a010::/64');
+
+        // /48 leaves 16 bits for the subnet ID until /64.
+        $this->assertCalculatedPrefix('2001:db8:1234::/48', '16', '2001:db8:1234:10::/64');
+        $this->assertCalculatedPrefix('2001:db8:1234::/48', '48879', '2001:db8:1234:beef::/64');
+    }
+
+    public function testPrefixLength()
+    {
+        // Empty range or 1 means a single /64 slot.
+        $this->assertUsablePrefixLength(56, '0', '', 64);
+        $this->assertUsablePrefixLength(56, '0', '1', 64);
+
+        // Larger ranges can be aggregated when starting at an aligned prefix ID.
+        $this->assertUsablePrefixLength(56, '0', '2', 63);
+        $this->assertUsablePrefixLength(56, '0', '4', 62);
+        $this->assertUsablePrefixLength(56, '0', '16', 60);
+
+        // Configuration stores decimal 16; GUI displays this as hexadecimal 10.
+        $this->assertUsablePrefixLength(56, '16', '16', 60);
+
+        // Decimal 10 is hexadecimal 0x0a and only aligned to 2 slots.
+        $this->assertUsablePrefixLength(56, '10', '16', 63);
+    }
+}