ZIP, EXE, DMG and ISO: the formats driver files came in
The 2010 index was full of file names ending in .zip, .exe and .dmg. Each of those endings tells you how the file is packaged and what your computer will do with it. Knowing the difference is basic file hygiene, and it is the best defence against the most common tricks played with downloaded software.
ZIP: a container
A ZIP file is an archive: a single file that holds one or more other files, usually compressed. Opening a ZIP does not run anything; it only shows or extracts the contents. Driver packages were often zipped so that a whole folder of files could be downloaded at once.
Be wary of a ZIP that contains a single program with a misleading name, such as "driver.pdf.exe". Windows hides known file extensions by default, so turning on "show file name extensions" in File Explorer is a simple safety step.
EXE and MSI: Windows programs
An EXE file is a Windows program; running it executes code with your user rights, and installers usually ask for administrator rights as well. An MSI file is a Windows Installer package. Both should come from a source you trust, and both can carry a digital signature. In the file's properties, the "Digital Signatures" tab shows who signed it.
DMG and PKG: Mac packages
A DMG is a macOS disk image. Opening it mounts a virtual disk, usually containing an app to drag into the Applications folder or a PKG installer. macOS checks downloaded apps with Gatekeeper, which warns about software that is not signed or not notarised by a registered developer. Old modem installers predate notarisation, which is one reason they trigger warnings today.
ISO: an image of a disc
An ISO file is a byte-for-byte image of an optical disc. USB modems used a related trick: they contained a small built-in "CD-ROM" holding the installer, which appeared as a drive when you plugged the stick in. That is where the name "zero-CD" or "autorun" mode comes from, and why Linux needs usb_modeswitch to switch the device into modem mode.
Summary
| Ending | What it is | Runs code when opened? |
|---|---|---|
| .zip | Compressed archive | No (but may contain programs) |
| .exe / .msi | Windows program / installer | Yes |
| .dmg | macOS disk image | No (contains apps or installers) |
| .pkg | macOS installer package | Yes, after you confirm |
| .iso | Disc image | No (mounted like a disc) |
Good habits
- Show file extensions, always.
- Prefer the maker's or operator's official source.
- Check signatures and, where published, checksums.
- Do not run installers from mirror sites, whatever they promise.
The same rules protect you with any app that holds a balance, whether a bank, a wallet or a sportsbook: install from the official store, keep it updated and ignore "special edition" installers passed around online.
