What is a URL (Uniform Resource Locator)?

Every URL consists of a formula and a formula-specific part: 

• Formula: the URL formula specifies both the kind of resource and the method needed to access it. The URL often has the same name as the accessor’s protocol at the ap­pli­ca­tion level. Common formulae are mailto, file, ftp or http/https

• Formula-specific part: depending on the kind of formula, the formula-specific part of the URL is made up of a number of segments that contain the resource’s location as well as optional pro­cess­ing pa­ra­me­ters.

The separator between the formula and the formula-specific part is a colon. Depending on the formula, you may also need two slashes, which were commonly used in the early days of the internet, but have no specific function today. 

A URL is based on the following URI-syntax:

Each segment of the formula specific part has its own function. The user, password, host and port sections are called “Au­thor­i­ties”. The authority indicates which computer a resource can be found on and what name is assigned to it.

• user and password: the user and password sections contain the username and password of the person au­tho­rized to access the resource and they are separated by a colon. Both details are only required if the resource requests au­then­ti­ca­tion. Username and password are separated by an @ sign from the host URI segment

• host: the URI host segment usually includes a Domain including a top, second- and third-level domain, in­di­cat­ing which specific host should retrieve the resource. Al­ter­na­tive­ly, the computer’s name can be specified in the form of an IP address

• port: by spec­i­fy­ing a port number, you can control a specific TCP/IP port in the network. Since most formulae already have a standard port, a separate entry is optional. For example, standard ports are 80 for HTTP, 443 for HTTPS or 21 for FTP. A port number should only be given if no general port is defined or if a non-default port is being used for standard trans­mis­sions. The port number is separated from the host section by a colon

The “authority” domain is usually specified in human-readable form. Computers, on the other hand, work with IP addresses. Visiting a website requires an in­ter­me­di­ate step, im­per­cep­ti­ble to the user: the name res­o­lu­tion based on the Domain Name System (DNS).

The URI’s authority is followed by an in­di­ca­tion of where the resource is located on the computer, as well as the optional com­po­nents: query string and fragment iden­ti­fi­er.

• path: the URI segment path contains the resource file reference and reveals its location on the target computer. The file path always starts with a slash (/)

• query: some websites contain ex­e­cutable com­po­nents and, in addition to the file path, expect a “query string” (also called a query part). This includes pa­ra­me­ters (such as user input) that are passed along with the URL and processed by the server. This is customary for dynamic web pages that are only created at the time of retrieval from database data records. The query string is always initiated with a question mark (?)

• fragment: if a specific location in a resource needs to be ref­er­enced, the URI ends with a fragment iden­ti­fi­er. This is separated with a hashtag (#) and usually refers to a label uniquely iden­ti­fied by an index in an HTML document – like a sub­head­ing, for example

The elements of URI syntax that contain a URL depends on the formula. The URL build is de­ter­mined by the type of resource. The following list includes the most common URL types:

A user who wants to access files in a remote file system using FTP usually has to au­then­ti­cate itself. Therefore, URLs that reference FTP resources usually contain access data (username and password).

The URL standard only supports a limited character set of selected American Code for In­for­ma­tion In­ter­change (ASCII) char­ac­ters. In addition, various char­ac­ters already have certain functions, like iden­ti­fy­ing in­di­vid­ual segments and sub­se­quent­ly allowing a URL to decompose or be processed.

The following char­ac­ters have already been assigned a specific function in the URL standard:

For example, the question mark (?) initiates a query string. Various pa­ra­me­ters in the query string are delimited with the ampersand (&). The separator between parameter name and value is the equal sign (=). The hash (#) initiates the jump label.

Char­ac­ters without a pre­de­fined function include all letters and digits and the special char­ac­ters mentioned below:

Other than the ASCII char­ac­ters listed here, non-ASCII char­ac­ters may now be used in URLs and must be rewritten. It is also possible to rewrite one of the reserved char­ac­ters to prevent it from being in­ter­pret­ed by its pre­de­fined meaning. To convert ASCII char­ac­ters, the URL standard uses the masking character % (percent) and the ASCII value table in hexa­dec­i­mal notation. Non-ASCII char­ac­ters are also rewritten using percent rep­re­sen­ta­tion. RFC 3986 rec­om­mends ASCII-com­pat­i­ble encoding based on UTF-8. This rec­om­men­da­tion is not binding and the service providers ul­ti­mate­ly decide which encoding is used. In contrast, domain special char­ac­ters are converted to ASCII-com­pat­i­ble strings using punycode. Learn more about encoding with punycode in our article on in­ter­na­tion­al domain names.

URLs can be absolute or relative URLs. Absolute URLs are uni­ver­sal­ly valid and include all segments required for the given formula. Relative URLs, on the other hand, are only valid in specific contexts and inherit certain prop­er­ties from them, so that cor­re­spond­ing URL sections become redundant and can be omitted. The in­for­ma­tion that context provides includes the protocol, domain or even path to the resource.

Relative URLs are used in webpage hy­per­links that lead to different subpages of a website. The link URL is the data from the webpage it’s leading to.

The following examples show a link from www.example.org/index/page1 to www.example.org/index/page2 with absolute or relative URLs.

Hyperlink with an absolute URL:

Hyperlink with a relative URL:

Relative URLs have the advantage that they are sig­nif­i­cant­ly shorter and con­tribute to a stream­lined, clear source code. In addition, hy­per­links with relative URLs fa­cil­i­tate domain re­lo­ca­tion. If a website domain changes, it must be exchanged manually with an internal link that has an absolute URL or redi­rect­ed using redirects. This effort is un­nec­es­sary for relative URLs that don’t have an “authority”, and thus, don’t need domain in­for­ma­tion.