19.1 Java4620: general information  (Page 5/8)

Theoretically, there are 65,535 available ports. Port numbers between 1 and 1023 are predefined to be used for certain standard services. For example, ifyou want to connect with server software that communicates using the HTTP protocol, you would normally connect to port 80 on the server of interest.

Similarly, if you want to connect to a port that will tell you the time, you should connect to port 13, assuming that you can find a server somewhere that iswilling to support port 13. If you want to connect to a port that will simply echo whatever you send to it (usually for test purposes) , you should connect to port 7. We will write Java applications that connect to all of theseports.

In the interest of brevity, I am not going to provide a complete list of ports and their services.However, you should be able to find all the information you might need about port numbers and the services they support by starting your favorite searchengine and searching for " well known ports "".

Firewalls

You may have heard about firewalls . A firewall is the common name given to the equipment and associated software that is used to insulate thenetwork inside of a company from the Internet at large outside the company. Typically, the firewall will restrict the degree to which computers inside thecompany can communicate with the Internet for security and other reasons.

Proxy Servers

You may also have heard about proxy servers . A proxy server acts as an interface between computers inside the company and the Internet at large.

Oftentimes the proxy server will have the ability to cache web pages for limited periods of time. For example, if ten people inside the companyattempt to connect to the same Internet server and download the same web page within a (hopefully) short period of time, that page may be saved on the proxy server on the first attempt and then delivered to the next nine peoplewithout re-acquiring it from the outside web server. This can significantly improve delivery time and reduce network traffic into and out of the company. Itcan also result in the delivery of stale pages in some cases.

Standards and Protocols

At some point, you may be interested in obtaining technical information about Internet standards and protocol specifications. A good place to start lookingfor such information is (External Link) .

URL

URL is an acronym for Uniform Resource Locator . (It is also the name of a class in Java.) A URL is a pointer to a particular resource at a particular location on the Internet. A URL specifiesthe following:

• the protocol used to access the server (such as http)
• the name of the server
• the port on the server (optional)
• the path and name of a specific file on the server (sometimes optional)
• the anchor or reference point within the file (optional)

Sometimes the name of the file can be omitted, in which case an HTTP browser will usually append the file name index.html to the specified path and try to load that file. For example, as of this writing, you can connect to myhome page on the HTTP server at Austin Community College using either of the following URLs: