diff --git a/util.hpp b/util.hpp
index 73649f0..ab7ea51 100644
--- a/util.hpp
+++ b/util.hpp
@@ -26,7 +26,7 @@
 constexpr auto localAdminMask = 0x020000000000;
 constexpr auto broadcastMac = 0xFFFFFFFFFFFF;
 
-constexpr auto format = "%hhx:%hhx:%hhx:%hhx:%hhx:%hhx";
+constexpr auto format = "%02hhx:%02hhx:%02hhx:%02hhx:%02hhx:%02hhx";
 constexpr size_t size = 18;
 
 /** @brief validate the mac address