Basic Network Concepts - Java Network Programming - page 14

Protocol Port Protocol Purpose

FTP 21 TCP This port is used to send FTP commands like put and get .

SSH 22 TCP Used for encrypted, remote logins.

Telnet 23 TCP Used for interactive, remote command-line sessions.

smtp 25 TCP The Simple Mail Transfer Protocol is used to send email between machines.

time 37 TCP/UDP A time server returns the number of seconds that have elapsed on the server since midnight,

January 1, 1900, as a four-byte, unsigned, big-endian integer.

whois 43 TCP A simple directory service for Internet network administrators.

finger 79 TCP A service that returns information about a user or users on the local system.

HTTP 80 TCP The underlying protocol of the World Wide Web.

POP3 110 TCP Post Office Protocol version 3 is a protocol for the transfer of accumulated email from the host to

sporadically connected clients.

NNTP 119 TCP Usenet news transfer; more formally known as the “Network News Transfer Protocol.”

IMAP 143 TCP Internet Message Access Protocol is a protocol for accessing mailboxes stored on a server.

dict 2628 TCP A UTF-8 encoded dictionary service that provides definitions of words.

The Internet
The Internet is the world's largest IP-based network. It is an amorphous group of com‐
puters in many different countries on all seven continents (Antarctica included) that
talk to one another using IP protocols. Each computer on the Internet has at least one
IP address by which it can be identified. Many of them also have at least one name that
maps to that IP address. The Internet is not owned by anyone, although pieces of it are.
It is not governed by anyone, which is not to say that some governments don't try. It is
simply a very large collection of computers that have agreed to talk to one another in a
standard way.

The Internet is not the only IP-based network, but it is the largest one. Other IP networks
are called internets with a little i : for example, a high-security internal network that is
not connected to the global Internet. Intranet loosely describes corporate practices of
putting lots of data on internal web servers that are not visible to users outside the local
network.

Unless you're working in a high-security environment that's physically disconnected
from the broader network, it's likely that the internet you'll be using is the Internet. To
make sure that hosts on different networks on the Internet can communicate with each
other, a few rules need to be followed that don't apply to purely internal internets. The
most important rules deal with the assignment of addresses to different organizations,
companies, and individuals. If everyone picked the Internet addresses they wanted at
random, conflicts would arise almost immediately when different computers showed
up on the Internet with the same address.