diff --git a/os/dev/slip.c b/os/dev/slip.c
index 6385a6e5e..17404bf1b 100644
--- a/os/dev/slip.c
+++ b/os/dev/slip.c
@@ -41,14 +41,13 @@
 #define SLIP_ESC_ESC 0335
 /*---------------------------------------------------------------------------*/
 PROCESS(slip_process, "SLIP driver");
-
-uint8_t slip_active;
-
+/*---------------------------------------------------------------------------*/
+static uint8_t slip_active;
 /*---------------------------------------------------------------------------*/
 #if 1
 #define SLIP_STATISTICS(statement)
 #else
-uint16_t slip_rubbish, slip_twopackets, slip_overflow, slip_ip_drop;
+static uint16_t slip_rubbish, slip_twopackets, slip_overflow, slip_ip_drop;
 #define SLIP_STATISTICS(statement) statement
 #endif
 /*---------------------------------------------------------------------------*/
diff --git a/os/dev/slip.h b/os/dev/slip.h
index 759de1a46..75ae58fd8 100644
--- a/os/dev/slip.h
+++ b/os/dev/slip.h
@@ -64,12 +64,6 @@ int slip_input_byte(unsigned char c);
 
 uint8_t slip_write(const void *ptr, int len);
 
-/* Did we receive any bytes lately? */
-extern uint8_t slip_active;
-
-/* Statistics. */
-extern uint16_t slip_rubbish, slip_twopackets, slip_overflow, slip_ip_drop;
-
 /**
  * Set a function to be called when there is activity on the SLIP
  * interface; used for detecting if a node is a gateway node.