diff --git a/webui/static/hoover/components/modal.js b/webui/static/hoover/components/modal.js
index 98806f6..6570d50 100644
--- a/webui/static/hoover/components/modal.js
+++ b/webui/static/hoover/components/modal.js
@@ -243,6 +243,7 @@ export function QuickModal(props = {}) {
                 ];
             }
             formModal(inner, title, fields, actions);
+            if (props.postRender) props.postRender(inner, data);
         });
     };
 }
diff --git a/webui/static/pages/dhcp.js b/webui/static/pages/dhcp.js
index 94431ce..93c106f 100644
--- a/webui/static/pages/dhcp.js
+++ b/webui/static/pages/dhcp.js
@@ -1,6 +1,6 @@
 import { html, PageHeader, ConfirmDelete, Tabs, Table, ServiceStatus, renderGuardMulti, esc, enc, $val, apiFetch, toast, definePage, getModel, ActionButton, ActionGroup, QuickModal } from '/static/hoover/index.js?v=7';
 
-function makeAddRange(activeZones) {
+function makeAddRange(activeZones, interfaces) {
     const opts = [
         ['', '(global)'],
         ...Object.entries(activeZones || {})
@@ -13,6 +13,27 @@ function makeAddRange(activeZones) {
             }),
     ];
 
+    const ranges = {};
+    (interfaces || []).forEach(iface => {
+        for (const cidr of (iface.ips || [])) {
+            if (cidr.indexOf('/') === -1) continue;
+            const parts = cidr.split('/');
+            const addr = parts[0].split('.').map(Number);
+            const prefix = parseInt(parts[1], 10);
+            if (prefix > 30 || prefix < 8) continue;
+            const mask = (0xFFFFFFFF << (32 - prefix)) >>> 0;
+            const netA = ((addr[0] << 24) + (addr[1] << 16) + (addr[2] << 8) + addr[3]) & mask;
+            const broadcast = (netA | (~mask >>> 0)) >>> 0;
+            const s = (netA + 100) >>> 0;
+            const e = Math.min((netA + 200) >>> 0, broadcast - 1);
+            const toIp = (n) => [n >> 24 & 0xFF, n >> 16 & 0xFF, n >> 8 & 0xFF, n & 0xFF].join('.');
+            if (s <= e) {
+                ranges[iface.name] = { start: toIp(s), end: toIp(e) };
+            }
+            break;
+        }
+    });
+
     return QuickModal({
         title: 'Add DHCP Range',
         fields: [
@@ -21,6 +42,23 @@ function makeAddRange(activeZones) {
             { label: 'End IP', id: 'r-end', placeholder: '192.168.1.200' },
             { label: 'Lease Time', id: 'r-lease', placeholder: '12h' },
         ],
+        postRender: (inner) => {
+            const select = inner.querySelector('#r-iface');
+            const startInput = inner.querySelector('#r-start');
+            const endInput = inner.querySelector('#r-end');
+            if (select && startInput && endInput) {
+                select.addEventListener('change', () => {
+                    const r = ranges[select.value];
+                    if (r) {
+                        startInput.value = r.start;
+                        endInput.value = r.end;
+                    } else {
+                        startInput.value = '';
+                        endInput.value = '';
+                    }
+                });
+            }
+        },
         submit: {
             url: '/api/dhcp/ranges',
             body: () => ({
@@ -131,7 +169,7 @@ export default definePage({
 
         const tabNames = ['ranges', 'leases', 'dns', 'active'];
         const actions = ActionGroup(
-            html`<button class="btn btn-primary" onClick=${() => makeAddRange(state.firewall.data?.zones?.active)(state)}>Add Range</button>`,
+            html`<button class="btn btn-primary" onClick=${() => makeAddRange(state.firewall.data?.zones?.active, state.firewall.data?.interfaces)(state)}>Add Range</button>`,
             html`<button class="btn btn-outline" onClick=${() => addLease(state)}>Static Lease</button>`,
             html`<button class="btn btn-outline" onClick=${() => addDns(state)}>DNS Record</button>`,
             ActionButton({