diff --git a/update-default-route.yml b/update-default-route.yml
index 96d9a00..f23207e 100644
--- a/update-default-route.yml
+++ b/update-default-route.yml
@@ -2,10 +2,25 @@
 - name: Update default gateway
   hosts: hosts
   become: true
+  vars:
+    new_gateway: "{{ gateway_ip }}"
 
   tasks:
+    - name: Validate gateway format
+      assert:
+        that:
+          - new_gateway is match("^(?:[0-9]{1,3}\.){3}[0-9]{1,3}$")
+        fail_msg: "Invalid IPv4 address provided."
+
+    - name: Replace gateway line in interfaces file
+      lineinfile:
+        path: /etc/network/interfaces
+        regexp: '^(\s*)gateway\s+'
+        line: "    gateway {{ new_gateway }}"
+        backrefs: yes
+
     - name: Apply new default route immediately
-      command: ip route replace default via 192.168.1.254
+      command: ip route replace default via {{ new_gateway }}
 
     - name: Restart networking
       systemd: