Internet Service Provider (ISP) cache
In order to speed up the internet browsing experience for their customers, each Internet Server Provider (ISP) caches DNS records. This means that after a customer of a particular ISP views your website or sends you an email, the ISP keeps a record of what name servers your domain name were delegated to and what zone records existed on those name servers. When a new customer from the same ISP attempts to view your website or email you, the ISP will find the cached record that it saved locally on their system instead of looking the records up on the Internet. This saves time and provides the user a much faster internet browsing experience.
The problem is that if you make a change to the name servers or zone records used with your domain name, an ISP may not notice these changes until a few hours later. In the time between your changes taking place and the ISP updating their servers, the ISPs customers will continue to follow the old cached copy of your records.
This delay and use of cached records can cause some visitors to see your old website while others see your new website or for some emails to be sent to your old email server while others are sent to your new server. As soon as the cache at all the ISPs servers are updated, all records fall back into sync and all visitors are taken to the same place.
Time To Live (TTL)
The TTL is the time period that the zone file tells other servers to cache zone records for. By setting the TTL to a shorter time period, users can shorten the propagation time for their DNS changes to be noticable. Shortening the TTL increases the number of requests sent to the DNS server and increases load and processing time however.
We allow users to modify and set the TTL for zone records that they add into the their zone file. We do not allow users to modify the TTL for zone records which we place into their zone file by default.
Modifying a zone records TTL then modifying the record right away will not increase the propgation time of your DNS changes. The new TTL needs to be cached at the ISP prior to the DNS change in order for you to take advantage of the shorter TTL. If you wish to speed up DNS propgation for your own personal zone records, it is recommended that you modify the TTL to a shorter time period many hours prior to actually modifying the DNS records.
In some cases, ISPs ignore the TTL settings and update their cached records every two to three days so setting the TTL to a short time period does not always guarantee quicker propagation.