When DNS records are added or updated, the change can take 12 to 72 hours to update across the internet; this period is called propagation. When your domain is opened in a web browser, the request is not going to the hosting server directly. It has to pass through several ISP (Internet Service Provider) nodes first, so your computer starts by checking the local DNS cache, where previously viewed information is stored. Afterward, the request is sent to your Internet Service Provider, and from there, to the hosting server. Each node will check its cache first, and because ISP's refresh their caching at different intervals, it can take some time for the changes you've made to reflect globally.
Propagation time can vary, but a good rule of thumb is 24-48 hours for changes to take effect across the internet.
Because propagation depends on the record's TTL setting, a visitor's ISP, and location, it is difficult to determine when propagation has finished without using a DNS resolution tool.
Some popular tools include: