Internet Basics - URLs, protocols, EMail, Addresses, DNS names

Internet Robustness Principle

Be liberal in what you accept, and conservative in what you send.


Understanding a URL/URI

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


Examples of common protocols in URL format: telnet:// news://

Components of a Universal Resource Locator (URL):

Note that the protocol and host name parts of a URL or EMail address are 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:


Examples of EMail addresses:

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

Machines connect with each other on the Internet using IP numbers, not the names that humans are accustomed to using. (Computers always prefer using numbers to names!) These numbers are called "IP" (Internet Protocol) numbers.

IP numbers are 32-bits long in the current (obsolete) IPv4 Internet and usually look like this when written in human-readable, "dotted-quad" form:,, Each of the four numbers in a dotted-quad represents eight bits in the 32 bit address. (Hence, the maximum value for any number is 255.) These 32-bit IP addresses are transmitted in the various Internet Protocols in Big-Endian form: the Most Significant Byte is sent first.

IP numbers are 128-bits long in the new IPv6 Internet and usually look like this when written in human-readable (32-digit hexadecimal) form: 2001:0db8:85a3:0000:0000:8a2e:0370:7334. Wikipedia IPv6

To establish a connection between the machines, the friendly, human-readable names such as must be turned into IP numbers, or C000200Ah, for use by the computer. 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. "".  There is no fixed limit to the depth of the hierarchy; however, more than four levels is uncommon.

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. "".  The IP numbers for the IPV4 Internet written this way are often said to be in "dotted quad" or "dotted decimal" form.  Two machines on the same network will have numbers that differ only in the rightmost bits, such as and  Their DNS names might be  and, though there is no reason for the names to be similar or even owned by the same people.

Just because two machines are on the same IP network does not mean that their DNS names must be related (though they usually are).  Conversely, just because two DNS names appear to be related does not mean that their IP addresses are similar (though they often are).

DNS - Domain Name Servers

Name servers "cache" name requests for a period of time (the TTL or "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.

Naturally, since you need a working DNS server to resolve a host name to an IP address, you must know and specify the actual IP addresses of any DNS servers you want your computer to use.  (If you used a name for your DNS, how would your computer turn that name into an IP address to contact the DNS to resolve its own name?)