diff --git a/lib/url.c b/lib/url.c
index d0c64281e..b78c200fa 100644
--- a/lib/url.c
+++ b/lib/url.c
@@ -105,6 +105,7 @@ int curl_win32_idn_to_ascii(const char *in, char **out);
 #include "rawstr.h"
 #include "warnless.h"
 #include "non-ascii.h"
+#include "inet_pton.h"
 
 /* And now for the protocols */
 #include "ftp.h"
@@ -4495,8 +4496,19 @@ static CURLcode parse_remote_port(struct SessionHandle *data,
         portptr = NULL; /* no port number available */
     }
   }
-  else
+  else {
+#ifdef ENABLE_IPV6
+    struct in6_addr in6;
+    if(Curl_inet_pton(AF_INET6, conn->host.name, &in6) > 0) {
+      /* This is a numerical IPv6 address, meaning this is a wrongly formatted
+         URL */
+      failf(data, "IPv6 numerical address used in URL without brackets");
+      return CURLE_URL_MALFORMAT;
+    }
+#endif
+
     portptr = strrchr(conn->host.name, ':');
+  }
 
   if(data->set.use_port && data->state.allow_port) {
     /* if set, we use this and ignore the port possibly given in the URL */