Dynamic DNS can help ensure that DNS queries work even if the web service being sought has recently switched IP addresses.
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Many web properties, such as APIs or websites, run on internet connections that have their IP addresses changed frequently; this creates a problem if the operators of those properties want to give a hosted resource a specific domain name, which must then store an IP address in Domain Name System (DNS) records. Dynamic DNS (DDNS) is a service that keeps the DNS updated with a web property's correct IP address, even if that IP address is constantly being updated.
For example, if a web administrator is operating a small website with a domain name of www.example.com and an IP address of 192.0.2.0, anytime another user enters www.example.com into their browser, the DNS will direct them to the server at 192.0.2.0. If the admin's ISP dynamically changes the IP to 192.0.2.1, a dynamic DNS service can automatically update the admin's DNS records so that other users trying to visit www.example.com will now go to the correct IP address.
In the early days of the Internet, IP addresses rarely changed, which made management of domains a lot simpler. But the rapid growth of the web and home computers with Internet access created a shortage of available IP addresses. This led to the Dynamic Host Configuration Protocol (DHCP), which lets ISPs assign IPs to their users dynamically. ISPs will typically maintain a shared pool of IP addresses and assign or 'lease' them to users as needed, for the duration of their connection or until a maximum amount of time has been reached. Although the introduction of IPV6 alleviated the IP address shortage, ISPs still often use DHCP because it is more cost-efficient than providing static IPs.
Large enterprises that run major web services require their ISPs to give them unchanging or 'static' IP addresses so they can operate using standard DNS practices. In contrast, smaller services tend to see their IP addresses changed by their ISPs quite frequently, so they require a dynamic DNS solution to keep their DNS records up to date. These smaller services can include small business websites, personal websites, DVRs, and security cameras.
There are a number of companies who offer dynamic DNS services with varying features and technologies. One very common method of enabling dynamic DNS is by providing users with software which runs on their computer or router. This software communicates with the dynamic DNS service provider anytime the IP addresses provided by the ISP is updated, and the dynamic DNS provider in turn updates the DNS with those changes, providing almost instant updates.