diff --git a/instances/models.py b/instances/models.py
index 99883859..16d8df13 100644
--- a/instances/models.py
+++ b/instances/models.py
@@ -372,7 +372,7 @@ class CreateInstance(models.Model):
     meta_prealloc = models.BooleanField(default=False, blank=True)
     virtio = models.BooleanField(default=True)
     qemu_ga = models.BooleanField(default=False)
-    mac = models.CharField(max_length=17, blank=True)
+    mac = models.CharField(max_length=255, blank=True)  # one per NIC, comma separated
     add_cdrom = models.CharField(max_length=16)
     add_input = models.CharField(max_length=16)
     video = models.CharField(
diff --git a/instances/templates/create_instance_w2.html b/instances/templates/create_instance_w2.html
index 5af065be..fdd42491 100644
--- a/instances/templates/create_instance_w2.html
+++ b/instances/templates/create_instance_w2.html
@@ -413,6 +413,7 @@
                                 <!-- populated from javascript -->
                             </ul>
                             <input id="networks" type="hidden" name="networks" value=""/>
+                            <input id="custom_mac" type="hidden" name="mac" value=""/>
                             <select class="form-select" id="network-control" name="network-control" multiple="multiple">
                                 {% for network in networks %}
                                     <option value="{{ network }}">{{ network }}</option>
@@ -864,6 +865,7 @@
                 $('#networks').val(input_value);
                 let selected_list_html = '';
                 mac_array = [];
+                $('#custom_mac').val('');
                 net_counter = 0;
                 if (input_value != '') {
                     $.each(input_value.split(','), function (index, value) {
@@ -894,7 +896,8 @@
 
     function populate_mac(c){
         mac_array[c] = $('input[name="mac' + c + '"').val();
-        $('input[name="mac"').val(mac_array.join(","));
+        // the custom form's MACs only: the other create forms have their own field
+        $('#custom_mac').val(mac_array.join(","));
     }
 
     function get_cust_vols(compute_id, pool) {
diff --git a/instances/tests/test_create_instance.py b/instances/tests/test_create_instance.py
index 35534404..cbd7287b 100644
--- a/instances/tests/test_create_instance.py
+++ b/instances/tests/test_create_instance.py
@@ -157,12 +157,17 @@ class NicMacsTestCase(TestCase):
             with self.subTest(mac=mac), self.assertRaises(ValueError):
                 nic_macs(mac, networks)
 
-    def test_api_serializer_refuses_extra_macs(self):
-        serializer = CreateInstanceSerializer(data={
-            "name": "api-vm", "vcpu": 1, "vcpu_mode": "host-model", "memory": 512,
-            "networks": "default", "mac": "52:54:00:00:00:01,52:54:00:00:00:02", "nwfilter": "",
-            "storage": "default", "hdd_size": 1, "cache_mode": "none", "meta_prealloc": False,
-            "virtio": True, "qemu_ga": True, "video": "vga", "listener_addr": "0.0.0.0",
-        })
-        self.assertFalse(serializer.is_valid())
-        self.assertIn("mac", serializer.errors)
+    def test_api_serializer_takes_a_mac_per_network(self):
+        def serializer(networks):
+            return CreateInstanceSerializer(data={
+                "name": "api-vm", "vcpu": 1, "vcpu_mode": "host-model", "memory": 512,
+                "networks": networks, "mac": "52:54:00:00:00:01,52:54:00:00:00:02", "nwfilter": "",
+                "storage": "default", "hdd_size": 1, "cache_mode": "none", "meta_prealloc": False,
+                "virtio": True, "qemu_ga": True, "video": "vga", "listener_addr": "0.0.0.0",
+                "submission_key": "test-key",
+            })
+        two_nics = serializer("default,second")
+        self.assertTrue(two_nics.is_valid(), two_nics.errors)
+        extra = serializer("default")
+        self.assertFalse(extra.is_valid())
+        self.assertEqual(extra.errors["mac"], ["More MAC addresses than networks"])
diff --git a/instances/tests/test_create_page_js.py b/instances/tests/test_create_page_js.py
index 1f3d02f0..5d9c9b4a 100644
--- a/instances/tests/test_create_page_js.py
+++ b/instances/tests/test_create_page_js.py
@@ -1,6 +1,7 @@
 """Scripts of the VM create page, rendered on libvirt's test driver and run
 in Node against stand-ins."""
 
+import json
 import re
 import shutil
 import subprocess
@@ -12,6 +13,8 @@ from computes.models import Compute
 from django.contrib.auth import get_user_model
 from django.test import TestCase
 from django.urls import reverse
+from instances.forms import NewVMForm
+from lxml import html
 
 
 class CreatePageScriptTests(TestCase):
@@ -37,3 +40,21 @@ class CreatePageScriptTests(TestCase):
         result = subprocess.run(["node", "-e", script], capture_output=True, text=True, timeout=30)
         self.assertEqual(result.returncode, 0, result.stderr)
         self.assertEqual(result.stdout.strip(), reverse("buses", args=[compute.id, "i686", "pc", "cdrom"]))
+
+    @unittest.skipUnless(shutil.which("node"), "node is not installed")
+    def test_the_custom_form_sends_its_own_macs_and_leaves_the_others_alone(self):
+        compute, page = self.page()
+        custom = html.fromstring(page).xpath("//form[@aria-label='Create custom instance form']")[0]
+        self.assertEqual(custom.xpath(".//input[@name='mac']/@id"), ["custom_mac"])
+        function = re.search(r"function populate_mac\(.*?\n    }\n", page, re.S).group(0)
+        # the stand-in $ records each write; a MAC field holds what its selector names
+        script = "const writes = []; let mac_array = [];\n" \
+                 "const $ = sel => ({val: v => v === undefined ? sel.replace(/\\D/g, '') : writes.push([sel, v])});\n" + \
+                 function + "populate_mac(0); populate_mac(1); console.log(JSON.stringify(writes));"
+        result = subprocess.run(["node", "-e", script], capture_output=True, text=True, timeout=30)
+        self.assertEqual(result.returncode, 0, result.stderr)
+        self.assertEqual(json.loads(result.stdout), [["#custom_mac", "0"], ["#custom_mac", "0,1"]])
+
+    def test_a_mac_per_nic_fits_the_form(self):
+        macs = ",".join(f"52:54:00:00:00:{n:02x}" for n in range(8))
+        self.assertEqual(NewVMForm.base_fields["mac"].clean(macs), macs)