In blogging, a ping is an XML remote procedure call (a form of a message) by which a blog informs one or more servers that its content has been published or modified. Ping was first used by Dave Winer on weblogs.com in 2001 and that standard is used still today.
Open ping servers allow all kind of web services to subscribe to the list of blogs that have recently pinged them. Search engines can provide up-to-date results very quickly and with minimal effort by visiting blogs that have pinned a ping server. Similarly, syndication feed aggregators use lists provided by ping servers to inform their subscribers of updates to the blogs they follow.
There are also private ping servers that collect information for their environment. Most major blog search engines operate such servers. Unlike open ping servers, private ping servers do not share the information with the other web services that may compete with the services they feed. Since these servers do not share the information they have, bloggers have to ping a large number of servers.
WordPress sends this kind of request:
And it receives this kind of response (we can not see it):
<?xml version="1.0" ?>
<string>Thanks for the ping.</string>
Where Do We Have to Add This List?
/wp-admin/options-writing.php on a browser when logged in. You’ll get this list:
Where is the List of Ping Sites?
As the system is primitive, you need to manually check whether the sites presently offer the service:
As this is a GitHub gist, you can suggest new sites.
How to Add New Sites Avoiding Duplicate Entry?
Install Sublime Text. It is free to use. Paste the URLs of ping sites you got from various websites. Go to Menu > Edit > Permute Lines > Unique. This process will prune duplicate entries.
Do not forget to manually check whether the sites are working.