What is the difference between `Allow: /` & `Disallow: ` in robots.txt?

Question

What is the difference between the two robots.txt files below?

User-agent: *
Allow: /

vs.

User-agent: *
Disallow: 

---

On Wikipedia it lists the later as an example under the Examples section.

However later it has code similar to the first code:

User-agent: bingbot
Allow : /
Crawl-delay: 10

Answer 1

You should prefer to use the disallow syntax:

User-agent: *
Disallow:

Disallow is part of the original robots.txt standard that is understood by every bot that obeys robots.txt.

Allow is extension syntax introduced by Google and understood by a few bots only. It was added to be able to disallow everything but then re-allow a few things. It would be most appropriate to use it like:

User-agent: *
Disallow: /
Allow: /public

In that case, most bots wouldn't be able to crawl the site at all, but the few bots that understand Allow: would be able to crawl the public directory.

When Disallow: and Allow: directives conflict (as in the above example), the longer one that applies to a given URL takes precedence. Eg /public/foo would use the Allow: /public rule because both rules could apply but that rule is longer. /private/foo would use the Disallow: / rule because only it matches. The order of rules makes no difference.

Answer 2

The first one tells all user-agents such as web-crawlers or google index bots that they are allowed to explore all the website since / is the root path of the website, for example, http://example.org would be /, and https://example.org/admin would be /admin in your robots.txt

The Disallow directive does exactly the opposite, it tells the user-agents to stay out of said paths.

Allow and Disallow can be used in different ways, such as a whitelist or a blacklist.

And because of that, the following

User-agent: *
Allow: /

is the same as

User-agent: *
Disallow: 

The easiest way of understanding this is by thinking that Allow and Disallow are like "lists" of paths, but just one type of directive ( Allow, or Disallow ) should be used.

For example, let's blacklist our robots.txt using the Disallow directive, only denying the bing indexer to index our website.

User-agent: Bingbot
Disallow: /

User-agent: *
Disallow:

In short.

If disallow is empty, then everything is allowed.

And if you allow everything, then nothing is disallowed.