The most common type of domain names are hostnames that provide more memorable names to stand in for numeric IP addresses. They allow for any service to move to a different location in the topology of the Internet (or an intranet), which would then have a different IP address.
By making possible the use of unique alphabetical addresses instead of numeric ones, domain names allow Internet users to easily find and communicate with web sites and other server-based services. The flexibility of the domain name system allows multiple IP addresses to be assigned to a single domain name, or multiple domain names to be assigned to a single IP address. This means that one server may have multiple roles (such as hosting multiple independent Web sites), or one role can be spread among many servers. One IP address can even be assigned to several servers, such as with anycast and hijacked IP space.
Hostnames are restricted to using only the ASCII letters "a" through "z' (case-insenstive), the digits "0" through "9" and the hyphen, along with a few other restrictions. Registrars restrict the domains that they will allow to be registered to valid hostnames since, otherwise, they would be useless. The Internationalized domain name (IDN) system has been developed to bypass these restrictions on what characters can be used in hostnames, making it easier for non-english speakers to use the Internet. The underscore character is frequently used to ensure that a domain name is not recognized as a hostname, for example with the use of SRV records, although some older systems, such as Netbios allowed it. Due to confusion and other reasons, domain names with underscores in them are sometimes used where hostnames are required.
By making possible the use of unique alphabetical addresses instead of numeric ones, domain names allow Internet users to easily find and communicate with web sites and other server-based services. The flexibility of the domain name system allows multiple IP addresses to be assigned to a single domain name, or multiple domain names to be assigned to a single IP address. This means that one server may have multiple roles (such as hosting multiple independent Web sites), or one role can be spread among many servers. One IP address can even be assigned to several servers, such as with anycast and hijacked IP space.
Hostnames are restricted to using only the ASCII letters "a" through "z' (case-insenstive), the digits "0" through "9" and the hyphen, along with a few other restrictions. Registrars restrict the domains that they will allow to be registered to valid hostnames since, otherwise, they would be useless. The Internationalized domain name (IDN) system has been developed to bypass these restrictions on what characters can be used in hostnames, making it easier for non-english speakers to use the Internet. The underscore character is frequently used to ensure that a domain name is not recognized as a hostname, for example with the use of SRV records, although some older systems, such as Netbios allowed it. Due to confusion and other reasons, domain names with underscores in them are sometimes used where hostnames are required.