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What is Propagation
When DNS Records are added or updated, the change can take up to 48 hours to take effect due to caching. 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 local DNS cache. Afterwards, 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 changes you've made to reflect globally.
Propagation is dependent upon the record's TTL setting, a visitor's ISP, and their location. Because of this, it is difficult to determine when propagation has finished without the use of a DNS resolution tool. Some popular tools include: