URLs were designed to be portable. They were also designed to uniformly name all the resources on the Internet, which means that they will be transmitted through various protocols. Because all of these protocols have different mechanisms for transmitting their data, it was important for URLs to be designed so that they could be transmitted safely through any Internet protocol.
Safe transmission means that URLs can be transmitted without the risk of losing information. Some protocols, such as the Simple Mail Transfer Protocol (SMTP) for electronic mail, use transmission methods that can strip off certain characters. To get around this, URLs are permitted to contain only characters from a relatively small, universally safe alphabet.
In addition to wanting URLs to be transportable by all Internet protocols, designers wanted them to be readable by people. So invisible, nonprinting characters also are prohibited in URLs, even though these characters may pass through mailers and otherwise be portable.
To complicate matters further, URLs also need to be complete. URL designers realized there would be times when people would want URLs to contain binary data or characters outside of the universally safe alphabet. So, an escape mechanism was added, allowing unsafe characters to be encoded into safe characters for transport.
This section summarizes the universal alphabet and encoding rules for URLs.
Default computer system character sets often have ...