Understanding a URL
URL syntax for a Network (Internet-style) item:
Examples of common protocols:
Components of a Universal Resource Locator (URL):
- http: HyperText Transfer Protocol (standard Web pages)
- ftp: File Transfer Protocol (for downloading/uploading
- telnet: for direct terminal-to-terminal connections to
- 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
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
Understanding an Internet E-Mail address
Syntax for an Internet email address:
Examples of EMail addresses:
Note that the host name part of an EMail address is not
The user_id part of the EMail address may or may not be
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:
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 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 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. "220.127.116.11". 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 18.104.22.168
Their DNS names might be freenet1.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.