diff --git a/content/notes/24-protection-of-file-systems.md b/content/notes/24-protection-of-file-systems.md
index 6c67d2103..882cdf9c5 100644
--- a/content/notes/24-protection-of-file-systems.md
+++ b/content/notes/24-protection-of-file-systems.md
@@ -23,5 +23,20 @@ types of file access users can have
 	- if a directory is not executable you cannot get into it (run it) 
 
 # Access control bits
+three classes for each file
+- owner: user who created the file
+- group : set of users who are sharing the file: a group is defined by the systems
+- others: all other in the system
 
+when a file is create it has an owner id and a group id to identify the users of the file 
+
+3 permission bits for each class
+- r: readable
+- w: writeable
+- x: executble
+
+advanced file attributes
+- setuid bit: if set for executable file, regardless of who runs the program, it should be run with the priviledge of the owner of the file
+- setgid bit: same as setuid but run with priviledge of group
+- sticky bit: a directory with this bit set restricts the deletionof files within it
 # access control models
\ No newline at end of file