The World Wide Web would be a tangled web indeed if it were not for a well-defined system for locating the various resources that web browsers display as websites. Web addresses--known as uniform resource locators or URLs--are standardized, multipart, structured identifiers that web browsers use to specify how, where and what to find and retrieve from the web.
The parts of a URL comprise strings of letters, numbers and symbols that conform to a standardized naming convention and syntax. The URL format that is most familiar to web surfers is "http://www.hostname.com:80/directorypath/file.html." This URL format specifies the protocol, domain name, port number and the resource specification. The colons (:), slashes (/) and periods (.) separate the parts of a URL.
The protocol part of a URL specifies the Internet service scheme used to request and return the identified resource. HTTP--or hypertext transfer protocol--is the standard protocol for web servers. Other protocols include secure HTTP (https), file transfer protocol (ftp), network news transfer protocol (nntp) and simple mail transfer protocol (smtp). If the protocol part of a URL is not specified, web browsers use http as the default.
Domain Name Part
The domain name part of a URL--www.hostname.com, for example---is best understood by reading the sub-parts from right to left. The familiar domain type "com" stands for "company/commercial." Other common domain types include "org" for "organization;" "net" for "network;" and "gov" for "government." Country-specific domain types include "jp" for "Japan" and "uk" for "United Kingdom." The "hostname" is a sub-domain the "com" domain name and identifies a particular Internet server. The "www" identifies the World Wide Web sub-domain of "hostname.com." An explicit numeric internet protocol (IP) address could be used as a substitute for the entire domain part of a URL. The Internet's domain name system (DNS) translates domain names into IP addresses.
Port Number Part
The port number part of a URL identifies the specific communications endpoint for Internet protocols. By default, port 80 is the port number used for the HTTP protocol on the World Wide Web and does not usually need to be specified in a URL.
Resource Specification Part
The resource specification part of a URL identifies the location of the requested resource on the host server in the domain. The resource specification typically includes one or more sub-directory names, the name of the resource and the resource type. The resource name and type are often a file name with an extension suffix that identifies the type of file. The suffix ".htm" or ".html" identifies the file type of "hypertext markup language," the file type that provides the signature hyperlink functionality of web browsing. Another common type of resource is a dynamic query specification that passes query parameters to a service on the host server which returns the results as a web page to the web browser. Internet search engines use this type of URL which can be recognized by the question marks (?) in the resource specification part of the URL.
The uniform resource identifier (URI) is the preferred term used in technical literature to refer to standardized Internet naming and addressing technology. URI is a generic term that includes URLs as well as uniform resource names (URNs), uniform resource citations (URCs) and other classes of Internet resource identifiers.