diff --git a/src/filo-fs-ext2.ads b/src/filo-fs-ext2.ads
index 7c1e467..5537507 100644
--- a/src/filo-fs-ext2.ads
+++ b/src/filo-fs-ext2.ads
@@ -86,6 +86,16 @@
       Inline      : Inode_Extents := (others => 16#00#);
    end record;
 
+   type Group_Index is new Natural range 0 .. Natural (Inode_Index'Last / 2);
+   subtype Group_In_Block_Count is Group_Index
+      range 1 .. Group_Index (Block_Size'Last / Desc_Size'First);
+   type Desc_In_Block_Index is new Natural range 0 .. Natural (Group_In_Block_Count'Last - 1);
+
+   type Inode_In_Group_Index is new Natural range 0 .. Natural (Inode_Index'Last / 2);
+   subtype Inode_In_Block_Count is Inode_In_Group_Index
+      range 1 .. Inode_In_Group_Index (Block_Size'Last / Inode_Size'First);
+   type Inode_In_Block_Index is new Natural range 0 .. Natural (Inode_In_Block_Count'Last - 1);
+
    type Mount_State is record
       Part_Len             : Partition_Length := 0;
       First_Data_Block     : FSBlock_Offset := 0;
@@ -93,7 +103,9 @@
       Block_Size           : Ext2.Block_Size := Ext2.Block_Size'First;
       Inodes_Per_Group     : Inode_Index := Inode_Index'First;
       Inode_Size           : Ext2.Inode_Size := Ext2.Inode_Size'First;
+      Inodes_Per_Block     : Inode_In_Block_Count := Inode_In_Block_Count'First;
       Desc_Size            : Ext2.Desc_Size := Ext2.Desc_Size'First;
+      Desc_Per_Block       : Group_In_Block_Count := Group_In_Block_Count'First;
       Feature_Extents      : Boolean := False;
       Feature_64Bit        : Boolean := False;
    end record;