When you change your domain's DNS settings, it can take some time until the changes are visible everywhere.
Time to Live for DNS Changes
To increase the speed at which a domain is resolved to IP addresses or the speed of other DNS queries for a domain, your domain's DNS settings are temporarily saved on a variety of servers. The TTL (time to live) specifies how long a server may temporarily save a domain's DNS settings. A shorter TTL means that changes take effect more quickly, while a longer TTL provides faster speeds when your website is accessed.
TTL Settings at 1&1
1&1 uses a time to live of one hour for all DNS settings. Therefore, changes to DNS settings are usually available everywhere within an hour.
One exception is when name servers are changed, as these are reported to the registry and have to be saved in the registry's root zone. The time required for a name server change to take effect everywhere therefore depends on the particular registry. Most registries update their zone very frequently, often within a few minutes. However, there are also registries that update their zone only once every 48 hours to reduce the load on their servers.
Influence of the Internet Service Provider
Some internet service providers ignore the TTL of the DNS settings in order to reduce the load on their servers and increase users' surfing speed. Instead, they update the DNS records only once every two to three days. If this is the case, it can take longer for the changes to take effect. Unfortunately, 1&1 has no control over these factors.