diff --git a/include/ruby/io/buffer.h b/include/ruby/io/buffer.h
index 0ee0a005e8..d5f646cd49 100644
--- a/include/ruby/io/buffer.h
+++ b/include/ruby/io/buffer.h
@@ -27,20 +27,24 @@ RUBY_EXTERN size_t RUBY_IO_BUFFER_DEFAULT_SIZE;
 
 enum rb_io_buffer_flags {
     // The memory in the buffer is owned by someone else.
-    RB_IO_BUFFER_EXTERNAL = 0,
+    // More specifically, it means that someone else owns the buffer and we shouldn't try to resize it.
+    RB_IO_BUFFER_EXTERNAL = 1,
     // The memory in the buffer is allocated internally.
-    RB_IO_BUFFER_INTERNAL = 1,
+    RB_IO_BUFFER_INTERNAL = 2,
     // The memory in the buffer is mapped.
-    RB_IO_BUFFER_MAPPED = 2,
+    // A non-private mapping is marked as external.
+    RB_IO_BUFFER_MAPPED = 4,
 
     // The buffer is locked and cannot be resized.
-    RB_IO_BUFFER_LOCKED = 16,
+    // More specifically, it means we can't change the base address or size.
+    // A buffer is typically locked before a system call that uses the data.
+    RB_IO_BUFFER_LOCKED = 32,
 
     // The buffer mapping is private and will not impact other processes or the underlying file.
-    RB_IO_BUFFER_PRIVATE = 32,
+    RB_IO_BUFFER_PRIVATE = 64,
 
     // The buffer is read-only and cannot be modified.
-    RB_IO_BUFFER_IMMUTABLE = 64
+    RB_IO_BUFFER_IMMUTABLE = 128
 };
 
 enum rb_io_buffer_endian {
diff --git a/io_buffer.c b/io_buffer.c
index 63f5bd2c24..d87575bfdd 100644
--- a/io_buffer.c
+++ b/io_buffer.c
@@ -83,6 +83,9 @@ io_buffer_map_file(struct rb_io_buffer *data, int descriptor, size_t size, off_t
     if (flags & RB_IO_BUFFER_PRIVATE) {
         access |= FILE_MAP_COPY;
         data->flags |= RB_IO_BUFFER_PRIVATE;
+    } else {
+        // This buffer refers to external data.
+        data->flags |= RB_IO_BUFFER_EXTERNAL;
     }
 
     void *base = MapViewOfFile(mapping, access, (DWORD)(offset >> 32), (DWORD)(offset & 0xFFFFFFFF), size);
@@ -107,6 +110,8 @@ io_buffer_map_file(struct rb_io_buffer *data, int descriptor, size_t size, off_t
         data->flags |= RB_IO_BUFFER_PRIVATE;
     }
     else {
+        // This buffer refers to external data.
+        data->flags |= RB_IO_BUFFER_EXTERNAL;
         access |= MAP_SHARED;
     }
 
@@ -572,7 +577,7 @@ rb_io_buffer_external_p(VALUE self)
     struct rb_io_buffer *data = NULL;
     TypedData_Get_Struct(self, struct rb_io_buffer, &rb_io_buffer_type, data);
 
-    return data->flags & (RB_IO_BUFFER_INTERNAL | RB_IO_BUFFER_MAPPED) ? Qfalse : Qtrue;
+    return data->flags & RB_IO_BUFFER_EXTERNAL ? Qtrue : Qfalse;
 }
 
 static VALUE
@@ -611,12 +616,6 @@ rb_io_buffer_immutable_p(VALUE self)
     return data->flags & RB_IO_BUFFER_IMMUTABLE ? Qtrue : Qfalse;
 }
 
-static int
-io_buffer_external_p(enum rb_io_buffer_flags flags)
-{
-    return !(flags & (RB_IO_BUFFER_INTERNAL | RB_IO_BUFFER_MAPPED));
-}
-
 VALUE
 rb_io_buffer_lock(VALUE self)
 {
@@ -858,7 +857,7 @@ rb_io_buffer_resize(VALUE self, size_t size)
         return;
     }
 
-    if (io_buffer_external_p(data->flags)) {
+    if (data->flags & RB_IO_BUFFER_EXTERNAL) {
         rb_raise(rb_eIOBufferMutationError, "Cannot resize external buffer!");
     }
 
diff --git a/test/ruby/test_io_buffer.rb b/test/ruby/test_io_buffer.rb
index f2b9ca45e1..2b371f873d 100644
--- a/test/ruby/test_io_buffer.rb
+++ b/test/ruby/test_io_buffer.rb
@@ -18,14 +18,14 @@ class TestIOBuffer < Test::Unit::TestCase
   end
 
   def test_flags
-    assert_equal 0, IO::Buffer::EXTERNAL
-    assert_equal 1, IO::Buffer::INTERNAL
-    assert_equal 2, IO::Buffer::MAPPED
+    assert_equal 1, IO::Buffer::EXTERNAL
+    assert_equal 2, IO::Buffer::INTERNAL
+    assert_equal 4, IO::Buffer::MAPPED
 
-    assert_equal 16, IO::Buffer::LOCKED
-    assert_equal 32, IO::Buffer::PRIVATE
+    assert_equal 32, IO::Buffer::LOCKED
+    assert_equal 64, IO::Buffer::PRIVATE
 
-    assert_equal 64, IO::Buffer::IMMUTABLE
+    assert_equal 128, IO::Buffer::IMMUTABLE
   end
 
   def test_endian