ISO 9660 file signature
Magic number at offset 32769 · CD001
An ISO 9660 optical disc image places the identifier CD001 at offset 32769 (0x8001), the start of the primary volume descriptor after the 32 KB system area. It may repeat at 0x8801 and 0x9001.
What this magic number means
A magic number is a short, fixed run of bytes at a known position that tells a program what a file really is, no matter what the filename says. If a file contains, at offset 32769, the bytes 43 44 30 30 31 (the text CD001), it is a ISO 9660 file.
Because the marker sits 32 KB into the file, the first 32768 bytes are usually zero or a boot sector, which is why a hex dump of just the start of an ISO can look empty.
Formats that use the 43 44 30 30 signature
These file types in our database carry this signature:
How to check a file's signature
You can read the first bytes of any file yourself. The magic bytes are shown in hex, the same way this page lists them.
Linux & macOS
- xxd -s 32769 -l 5 example.iso
- hexdump -C -s 32769 -n 5 example.iso
- file example.iso
Windows (PowerShell)
- Format-Hex -Path example.iso -Count 32774
Python
- f=open("example.iso","rb");f.seek(32769);f.read(5).hex()
In your browser
- Drop the file into the WhatFileType identifier, which reads the signature without uploading it.
Frequently asked questions
What is the ISO 9660 file signature?
ISO 9660 files carry, at offset 32769, the hex bytes 43 44 30 30 31 (CD001 in ASCII). This magic number identifies the format regardless of the file's name or extension.
How do I check a file's magic number?
Open the file in a hex editor, or run a command such as xxd -l 32774 example.iso on Linux or macOS, or Format-Hex -Path example.iso -Count 32774 in Windows PowerShell, and read the first bytes.
Can a file fake the ISO 9660 signature?
Renaming a file does not change its bytes, so the extension can lie but the signature usually cannot. A genuine ISO 9660 file has these exact bytes; a file with the wrong bytes is not really ISO 9660, whatever its name says.