diff --git a/include/net.h b/include/net.h
index dd673f0ece82188094744b1eb12f84b6a465276e..5e3495bd75c8207fa40de5f813d767470fd48158 100644
--- a/include/net.h
+++ b/include/net.h
@@ -520,7 +520,7 @@ extern IPaddr_t string_to_ip(char *s);
 extern void	VLAN_to_string (ushort x, char *s);
 
 /* Convert a string to a vlan id */
-extern ushort string_to_VLAN(char *s);
+extern ushort string_to_VLAN(const char *s);
 
 /* read a VLAN id from an environment variable */
 extern ushort getenv_VLAN(char *);
diff --git a/net/net.c b/net/net.c
index 7576419b7a3bbe232897cc66bc074201a3ab4207..a6096324126ada55e2f30afe023d376f6720ff16 100644
--- a/net/net.c
+++ b/net/net.c
@@ -1913,7 +1913,7 @@ void VLAN_to_string(ushort x, char *s)
 		sprintf(s, "%d", x & VLAN_IDMASK);
 }
 
-ushort string_to_VLAN(char *s)
+ushort string_to_VLAN(const char *s)
 {
 	ushort id;