How the Internet Works



The Internet is a network of computer networks connected together which
pass data among and within each other using special protocols. Within
each network, different computers are designated to handle certain types
of data, and we identify these computers according to these functions:

LAN

Host Computers Computers which you log into to send and access data to
and from other computers. 'Local hosts' are computers which you can
physically access (like your PC/ Mac) and 'remote hosts' are computers
which you can access only with the aid of some terminal software. 'Local
hosts' can also refer to those computers which are within a local
network, as opposed to computers in a foreign network which is hence
known as 'remote hosts'.

Servers Computers which specialise in sending, receiving and storing
data using a certain protocol. The data in servers are accessed by
software on local or remote hosts known as 'clients'.

Routers Computers which direct data traffic between networks.



Host Names and Addresses

All computers in the Internet have a unique host name. Technically, a
'host name' is just a name given to a computer, like 'leonis'. But in
order for this computer to be recognised by other Internet computers, it
needs to be given an 'Internet Protocol (IP) Address', or a 'Fully
Qualified Domain Name' (FQDN), or simply 'Domain Name' or 'Host Address'
for short.

IP Address An IP Address is made up of a series of numbers separated by
periods (e.g., 137.132.1.18). In order to be recognised by any Internet
computer, a computer needs to be assigned an IP Address. Your PC/ Mac
needs to be assigned an IP Address too if it is to access any Internet
host or server.

FQDN IP Addresses are usually mapped to a corresponding FQDN (which is
much easier to remember). For example, the computer with the IP Address
'137.132.1.18' is mapped to the FQDN 'leonis.nus.sg'. An FQDN is made up
of higher level domain names and the host name. The domain name 'sg'
indicates the computer belongs to a system in Singapore, and the domain
name 'nus' indicates the computer belongs to NUS. In order to access
Internet computers, you should know these host addresses.





In order for Internet computers to recognise and transmit data between
each other, these data must be sent via special protocols. A protocol is
a formal description of message formats and the rules two computers mus
follow to exchange those messages.

Types of Protocols

The following are some examples of commonly used protocols:

Internet Protocol (IP) The network-layer protocol used by Internet
computers to handle data.

Transmission Control Protocol over Internet Protocol (TCP/IP) The common
way of referring to a type of transport and application protocols which
runs over IP.

File Transfer Protocol (FTP) The protocol used to copy files from one
host to another in the Internet.

HyperText Transmission Protocol (HTTP) The protocol used to retrieve
documents written in HyperText Markup Language, which form the part of
the Internet known as the World Wide Web.

Network News Transfer Protocol (NNTP) The protocol used to distribute
and retrieve articles posted in Usenet.

Point-to-Point Protocol (PPP) The protocol used to transmit data over
serial links.

Post Office Protocol (POP) The protocol which retrieves e-mail from the
e-mail server to allow users to read them from their own personal
computers.

Serial Line Internet Protocol (SLIP) The protocol for running IP over
serial lines, like telephone lines. This gives users direct access to
Internet computers, using SLIP.

Simple Mail Transfer Protocol (SMTP) The server-to-server protocol used
to exchange e-mail.