Internet Basics

Understanding a URL

URL syntax for a Network (Internet-style) item:

protocol://computer_address_or_name/pathname

Examples of common protocols:


http://idallen.com/
mailto:idallen@idallen.ca 
ftp://ftp.algonquincollege.com/
telnet://telnet.ncf.carleton.ca
news://news.idallen.com/algonquinc.interest.linux

Components of a Universal Resource Locator (URL):

http: HyperText Transfer Protocol (standard Web pages)
ftp: File Transfer Protocol (for downloading/uploading files)
telnet: for direct terminal-to-terminal connections to remote computers
gopher: a predecessor of HTTP, largely replaced by HTTP
news: Network News Transfer Protocol (NNTP) for Usenet-style discussion groups
mailto: Simple Mail Transfer Protocol (SMTP) for sending E-Mail

Note that the host name part of a URL or EMail address is not case-sensitive. The remainder of the URL or EMail address may or may not be case-sensitive.

Understanding an Internet E-Mail address

Syntax for an Internet email address:

user_id@computer_address_or_name

Examples of EMail addresses:

    idallen@idallen.ca
    aa610@FREENET.CARLETON.CA

Note that the host name part of an EMail address is not case-sensitive. The user_id part of the EMail address may or may not be case-sensitive.

Computer Addresses and Names

Most of the connecting done between machines on the Internet happens between computers labelled by numbers, not names. (Computers seem to always prefer using numbers to names!) These numbers are called "IP" (Internet Protocol) numbers. The numbers are 32-bits long and usually look like this when written in human-readable form: 216.239.39.104, 206.14.210.245

To establish a connection between the machines, the friendly, human-readable names must be turned into IP numbers. Tables of these name-to-number maps typically reside on computers that serve as Domain Name Servers (DNS). These DNS Servers translate human-readable names to IP numbers.

Computer Names

Computer names in the DNS are hierarchical, separated by periods, with the most specific components on the left and the most general, large domains on the right, e.g. "news.idallen.com". There is no fixed limit to the depth of the hierarchy; however, more than four levels is uncommon.

There are a fixed number of "top level" domains, e.g. .gov, .edu, .com, .org, .name, .biz, .ca, .hu, .jp, .us, etc. Many of these are reserved for exclusive use by the USA.
Each domain gets control of the naming scheme used inside it and can use any naming convention it likes, e.g. .com.au, .edu.au, .net.au, etc.
Name servers only need to know how to find the name servers of the next level in the name tree - no server needs to know the whole tree.

IP Numbers

IP numbers are also hierarchical; however, the most specific parts of the IP number are on the right, and the most general, large network components are on the left, e.g. "134.117.136.20". The IP numbers written this way are often said to be in "dotted quad" form. Two machines on the same network will have numbers that differ only in the rightmost bits, such as 134.117.136.20 and 134.117.136.24. Their DNS names might be freenet1.carleton.ca and freenet4.carleton.ca. Just because two machines are on the same IP network does not mean that their DNS names must be related (though they usually are).

DNS - Domain Name Servers

Name servers "cache" name requests for a period of time (the "time to live"). Subsequent requests for the same name are served from the cache until the entry expires, then a fresh request is made to update the cache. Adding or changing a DNS name can take several days to propagate through all the caches in the Internet.