Googlebot

Googlebot is the generic name for two types of
web crawlers used by Google Search:

• Googlebot Smartphone:
  a mobile crawler that simulates a user on a mobile device.
• Googlebot Desktop:
  a desktop crawler that simulates a user on desktop.

You can identify the subtype of Googlebot by looking at the
HTTP user-agent request header
in the request. However, both crawler types obey the same product token (user agent token) in
robots.txt, and so you cannot selectively target either Googlebot Smartphone or Googlebot
Desktop using robots.txt.

For most sites Google Search primarily
indexes the mobile version
of the content. As such the majority of Googlebot crawl requests will be made using the mobile
crawler, and a minority using the desktop crawler.

How Googlebot accesses your site

For most sites, Googlebot shouldn’t access your site more than once every few seconds on
average. However, due to delays it’s possible that the rate will appear to be slightly higher
over short periods. If your site is having trouble keeping up with Google’s crawling requests, you
can reduce the crawl rate.

When crawling for Google Search, Googlebot crawls the first 2MB of a
supported file type, and
the first 64MB of a PDF file. From a rendering perspective, each resource referenced in the HTML
(such as CSS and JavaScript) is fetched separately, and each resource fetch is bound by the same
file size limit that applies to other files (except PDF files).
Once the cutoff limit is reached, Googlebot stops the fetch and only sends the already downloaded
part of the file for indexing consideration. The file size limit is applied on the uncompressed
data.
Other Google crawlers, for example Googlebot Video and Googlebot Image, may have
different limits.

When crawling from IP addresses in the US, the timezone of Googlebot is
Pacific Time.

Other
technical properties of Googlebot
are described in the overview of Google’s crawlers.

Blocking Googlebot from visiting your site

Googlebot discovers new URLs to crawl primarily from links embedded in previously crawled pages.
It’s almost impossible to keep a site secret by not publishing links to it. For example, as soon
as someone clicks a link from your “secret” site to another site, your “secret” site URL may
appear in the referrer tag and can be stored and published by the other site in its referrer log.

If you want to prevent Googlebot from crawling content on your site, you have a
number of options. Remember
there’s a difference between crawling and indexing; blocking Googlebot from crawling
a page doesn’t prevent the URL of the page from appearing in search results:

• Prevent Googlebot from crawling a page? Use a
  robots.txt file.
• Don’t want Google to index a page? Use
  noindex.
• Prevent a page from being accessible at all by both crawlers or users? Use
  another method, such as password protection.

Blocking Googlebot affects Google Search (including Discover and all Google Search features), as
well as other products such as Google Images, Google Video, and Google News.

Verifying Googlebot

Before you decide to block Googlebot, be aware that the HTTP user-agent request
header used by Googlebot is often spoofed by other crawlers. It’s important to verify that a
problematic request actually comes from Google. The best way to verify that a request actually
comes from Googlebot is to
use a reverse DNS lookup
on the source IP of the request, or to match the source IP against the
Googlebot IP ranges.