Sites Sharing IP Addresses Isn’t a Problem For Google
It’s common practice for websites to share IP addresses and having an implementation like this won’t cause ranking drops or performance decreases.
Only Use Separate URLs For Similar Products if They Match Separate Specific Queries
It’s usually better to consolidate signals for different product variations by using one URL. You might want to split them out if they match specific user search intents though, as one main product page might be too generic for certain queries.
Splitting or Merging Websites May Change Rankings
If you’re moving a whole website to a different domain, Google can move the signals across for the whole site. But if you’re splitting off part of a site to a new domain, or merging multiple websites, then the rankings may change as Google has to revaluate the sites individually.
Treat Tag Pages Like Any Other Page & Only Noindex Low Quality Ones
John recommends treating tag pages like any other page on your site and to differentiate between useful tag pages and low quality tag pages by noindexing the low quality ones.
When Splitting a Category Page Into Two New Ones, Redirect to One of the New URLs & Update Internal Linking
When splitting a category page into two separate pages, John recommends redirecting the old URL to one of the new pages and then updating internal linking normally within your website’s structure.
Merging Internal or External Pages on One Topic Will Result in Higher Rankings
If separate sites that rank well already and both focus on the same topic or service merge, then they will see an increase in rankings as Google sees an even stronger page than before.
Linking to Every Page From a Site’s Homepage Will Stop Google Understanding Site Architecture
Linking to every page on a site from the homepage means that Google will lose the structure of the website as a whole, unless the site only has a small number of pages.
Google Requires Distinct Section on Site For Geotargeting to be Understood
Google requires that websites using geo-targeting have a dedicated section on the site for specific territories such as a separate subdirectory or subdomain.
Google Requires Multilingual Sites to Have Some Form of URL Differentiation
Anything that differentiates a URL can work for Google for multilingual sites with hreflang e.g. subdomains, subdirectories or parameters.