diff --git a/instance_id.cpp b/instance_id.cpp
new file mode 100644
index 0000000..52cd1d6
--- /dev/null
+++ b/instance_id.cpp
@@ -0,0 +1,25 @@
+#include "instance_id.hpp"
+
+#include <stdexcept>
+
+namespace pldm
+{
+
+uint8_t InstanceId::next()
+{
+    uint8_t idx = 0;
+    while (idx < id.size() && id.test(idx))
+    {
+        ++idx;
+    }
+
+    if (idx == id.size())
+    {
+        throw std::runtime_error("No free instance ids");
+    }
+
+    id.set(idx);
+    return idx;
+}
+
+} // namespace pldm