The Internet is one of the most exciting communications mediums to appear. Each day more individuals are coming online, only to be confused by the panoply of information available and the confusing new terms. Notes From The Net will continue to introduce you to features of the Internet, the terminology you'll encounter and some of the highlights to be found there. This month's column is in two parts. The first half of the column will dissect URLs and the second half highlights a few online medical journals.
URL, pronounced Earl, stands for Uniform Resource Locator. An URL is an address pointer to the location of a resource on the Internet. It specifies the host computer on which it is stored, the directory location, and the file name. An URL may appear as "http://www.nlm.nih.gov/". This is the text representation of an Internet Protocol (IP) address which would appear as (126.96.36.199). After the browser has the address, it does a name lookup, and then starts communicating with the remote computer. The Domain Name System (DNS) server is a special Internet server that sends your browser the IP address of the computer you are trying to reach. Occasionally you will see a message stating: "DNS Lookup Error." This indicates that the name you typed into your browser could not be resolved into a legitimate IP address. This could happen if either the domain name was entered incorrectly or the domain itself is no longer in existence. Assuming a good connection, when your browser communicates with the remote server, your own IP address is embedded in the outbound packets. This gives the server your address and allows the server to send information back to your browser.
The text name contains some clues about the origin
of the URL. Domain names frequently contain information about
the type of organization or the country of origin. Table 1 contains
some of the commonly seen domain specifications. Countries are
frequently present as 2 letter combinations such as es for Spain
and ch for Switzerland.
|org||non-government, non-commercial, non-academic site|
|su||Russia (formerly Soviet Union)|
Let's examine an URL appearing as "http://www.nlm.nih.gov/".
The first few letters define the type of resource and transfer
protocol for your browser to use. These include ftp (ftp:// file
transfer protocol), gopher (gopher://), mail (mailto://) and http
(http:// - HyperText Transport Protocol). The second part of the
URL gives the name or address of the machine that contains the
resource we are seeking. Occasionally you will see a Web address
in the form http://188.8.131.52/, although this is not the recommended
format. The text format is preferred and will give more clues
about where you are heading. Examples you may encounter include
nih, National Institute of Health and cdc, Center For Disease
Control. Let's look at another example "www.who.ch".
This is in three parts, each separated by a decimal point. The
first part, www, indicates a world wide web page. The second,
who, indicates the World Health Organization. The last part, ch,
is the country top level domain for Switzerland. The last part
of an URL may contain a file name and subdirectory on the host
computer. This is case specific and must be entered with upper
case and lower case being precisely how it is registered on the
|http||material on a World Wide Web server|
|file||file on your local system, sometimes on an ftp server|
|ftp||a file on an anonymous FTP server|
|gopher||material on a Gopher server|
|WAIS||material on a WAIS server|
|telnet||connection to another computer via telnet|
|mailto||link to send electronic mail via your browser|
After this discussion, you should no longer have a fear of URLs. One practical use of this information is a method of handling the server message "404 not found" which indicates that a file does not exist. When you obtain this server error, you can be certain that the domain name exists, but the file does not. You can then gradually strip away the end of the URL, until a useful response is obtained. For example the URL "https://www.drpribut.com/sports/bigtoe.html" will yield an error. If you strip the last part of the URL which contains the file name, you will then have: "https://www.drpribut.com/sports". This should then allow you to either see the directory structure or to automatically display a default file. This same technique can be used to explore higher up the directory tree by trying: "http://www.clark.net/pub/".
Now we can stop thinking about URLs and start surfing
some intriguing medical sites. Some of the best scientific journals
are headed for the web. The most renowned medical journal, the
New England Journal of Medicine began
a site on the web earlier this year. It is located at "http://www.nejm.org/".
Starting from January of 1996 you will find the tables of contents
of each issue, full text of correspondence, editorials, book reviews,
opinion pieces (Sounding Board), Molecular Medicine and the Case
Records of the Massachusetts General Hospital . Abstracts of original
articles and special articles are available on-line, along with
the names and institutional affiliations of the authors. The full
text may be ordered to be sent by mail or fax.
The Mayo Clinic has placed their Health Magazine
on-line in the form of the "Mayo Online Health Magazine"
A summer highlight includes an article about ticks
that may be seen at: "http://healthnet.ivi.com/hnet/9605/hl/ticks.htm".
The American Medical Association has a whole host
of journals available on-line. To register at
no fee visit: "http://www.ama-assn.org/register/register.htm".
Unfortunately not many of their nearly 20 journals offer full
text. On-line journals include the Journal of The American Medical
Association and the Archives of Internal Medicine. The tables
of contents, abstracts of major manuscripts, and search functions
are available on-line.
The text and hot links of this article will be available
at my Home Page at:
Published in APMA News, August 1996
Occasionally you may also receive e-mail from someone with an apparent address of firstname.lastname@example.org. It is possible to determine the domain name that represents the IP address. You may personally perform your own Name Server Lookup. This is done in Windows using Windows client software. The Ultimate Collection of Winsock Software (TUCOWS) is an excellent source of Windows client software. Look in the category "Ping & NS-Lookup" for software which will look up the name server. Macintosh software, may also perform this function. Appropriate Mac software may be found at the Info-Mac Archive,at MIT. Search on the term "lookup". On a Unix system, the command: "nslookup" followed by the IP address will work.
If you are operating from behind a Firewall or within the military system, you may need the
opposite. You might need to determine the IP address in dotted decimal notation. This can be done
on the web. The following location will do the lookup in either direction:
Name Server Lookup
The four part host number that you have seen contains some additional information. The first two numbers give what is called the Network number. The second two are the local host number. The first number reveals how large a system you are accessing. The lower numbers are the larger systems. IBM is number 9 and ATT has host number 12. Table 4 classifies the systems into three classes. The largest systems are Class A and the smallest are Class C.
|Class||First Number||May Contain Network of this size|
New England Journal of Medicine
Mayo Online Health Magazine