Editing Domain Name System (section)

===Other applications===
The Domain Name System includes several other functions and features.

Hostnames and IP addresses are not required to match in a one-to-one relationship. Multiple hostnames may correspond to a single IP address, which is useful in [[virtual hosting]], in which many web sites are served from a single host. Alternatively, a single hostname may resolve to many IP addresses to facilitate [[fault tolerance]] and [[load balancing (computing)|load distribution]] to multiple server instances across an enterprise or the global Internet.

DNS serves other purposes in addition to translating names to IP addresses. For instance, [[mail transfer agent]]s use DNS to find the best mail server to deliver [[e-mail]]: An [[MX record]] provides a mapping between a domain and a mail exchanger; this can provide an additional layer of fault tolerance and load distribution.

The DNS is used for efficient storage and distribution of IP addresses of block-listed email hosts. A common method is to place the IP address of the subject host into the sub-domain of a higher level domain name, and to resolve that name to a record that indicates a positive or a negative indication.

For example:
* The address {{IPaddr|203.0.113.5}} is block-listed. It points to {{mono|5.113.0.203.blocklist.example}}, which resolves to {{IPaddr|127.0.0.1}}.
* The address {{IPaddr|203.0.113.6}} is not block-listed and points to {{mono|6.113.0.203.blocklist.example}}. This hostname is either not configured, or resolves to {{IPaddr|127.0.0.2}}.
E-mail servers can query blocklist.example to find out if a specific host connecting to them is in the block list. Many such block lists, either subscription-based or free of cost, are available for use by email administrators and anti-spam software.

To provide resilience in the event of computer or network failure, multiple DNS servers are usually provided for coverage of each domain. At the top level of global DNS, thirteen groups of [[root name server]]s exist, with additional "copies" of them distributed worldwide via [[anycast]] addressing.

[[Dynamic DNS]] (DDNS) updates a DNS server with a client IP address on-the-fly, for example, when moving between ISPs or mobile [[Hotspot (Wi-Fi)|hot spots]], or when the IP address changes administratively.