A noteworthy consequence of this distributed and caching architecture is that changes to the DNS are not always immediately effective globally. This is best explained with an example: If an administrator has set a TTL of 6 hours for the host www.checkdns.net, and then changes the IP address to which www.checkdns.net resolves at 12:01pm, the administrator must consider that a person who cached a response with the old IP Address at 12:00pm will not consult the DNS server again until 6:00pm.

The period between 12:01pm and 6:00pm in this example is called caching time, which is best defined as a period of time that begins when you make a change to a DNS record and ends after the maximum amount of time specified by the TTL expires. This essentially leads to an important logistical consideration when making changes to the DNS: not everyone is necessarily seeing the same thing you're seeing. RFC1537 helps to convey basic rules for how to set the TTL.

Note that the term "propagation", although very widely used, is a poor term to describe the effects of caching. Specifically, it implies that [1] when you make a DNS change, it somehow spreads to all other DNS servers (instead, other DNS servers check in with yours as needed), and [2] that you do not have control over the amount of time the record is cached (you have complete control for all DNS records on your domain, except your NS records and any authoritative DNS servers that use your domain name).

Many people incorrectly refer to a mysterious 48 hour or 72 hour propagation time when you make a DNS change. When you change the NS records for your domain or the IP addresses for hostnames of authoritative DNS servers using your domain (if any), there can be a lengthy period of time before all DNS servers use the new information. This is because those records are handled by the zone parent DNS servers (for example, the .com DNS servers if your domain is example.com), which typically cache those records for 48 hours. However, those DNS changes will be immediately available for any DNS servers that do not have them cached. And, any DNS changes on your domain other than the NS records and authoritative DNS server names can be nearly instantaneous, if you choose for them to be (by lowering the TTL once or twice ahead of time, and waiting until the old TTL expires before making the change).