diff --git a/src/net/addr/subnet.cpp b/src/net/addr/subnet.cpp
new file mode 100644
index 0000000..36d6049
--- /dev/null
+++ b/src/net/addr/subnet.cpp
@@ -0,0 +1,18 @@
+#include <fmt/format.h>
+
+#include <stdplus/net/addr/subnet.hpp>
+
+#include <stdexcept>
+
+namespace stdplus::detail
+{
+
+void invalidSubnetPfx(std::size_t pfx)
+{
+    throw std::invalid_argument(fmt::format("Invalid subnet prefix {}", pfx));
+}
+
+template class Subnet46<In4Addr, uint8_t>;
+template class Subnet46<In6Addr, uint8_t>;
+
+} // namespace stdplus::detail