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Secure Sockets Layer (SSL) protocol
for encrypted HTTP traffic
Enables a hyperlink to access a file
on the local file system
ftp (File Transfer
Enables a hyperlink to download files
from remote systems
Invokes a mail program to enable a
hyperlink to send an addressed e-mail
Enables a hyperlink to open a telnet
session on a remote host
T ABLE D-3 Some Commonly Used URL Protocols
These are the common protocols, but a variety of new protocols and URL forms are
being debated all the time. We'll present a discussion of emerging URL forms toward the
end of this appendix.
Other Features of URLs
In addition to the protocol, server address, directory, and filename, URLs often include a
username and password, a port number, and potentially more. Some URLs, such as mailto,
might even contain a different form of information altogether, such as an e-mail address
rather than a server or filename.
Username and Password
FTP and telnet are protocols for authenticated services. It is also possible to make HTTP an
authenticated service if you password-protect a directory or file. Authenticated services can
restrict access to authorized users, and the protocols can require a username and password
as parameters. A username and password precede a server name; for example, ftp syntax
looks like
The password could be optional or unspecified in the URL, making the form simply:
Regardless of the protocol, we should avoid putting login identifiers and especially passwords
in URLs. If it is not specified and the resource is protected, let the server issue a challenge so
that users provide it directly.
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