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Introduction to sockets

Author: Chad Z. Hower

Homepage: http://www.atozedsoftware.com

also chosen not to cover higher-level protocols, such as FTP, World Wide Web, etc,

as I assume you're familiar with these (after all, you are on the Internet using the

World Wide Web as you read this).

There are several concepts that must be introduced first. As much as possible, the

following concepts will be likened to a real world concept you are likely familiar

with: a phone system.

Winsock ("Windows Sockets")

Winsock is a defined and documented standard API for programming network protocols.

Most commonly it is used to program TCP/IP, but can also be used to program Novell

(IPX/SPX) and other network protocols. Winsock is accessible as a DLL that is part

server at a time. If a process needs to talk to multiple servers, it creates

multiple clients.

Likening it to a phone call, a client would be the person who makes a call.

Server

A server is a process that answers incoming requests. A typical server handles

numerous requests from many clients simultaneously. Each connection from the server

to the client, however, is a separate socket.

Likening it to a phone call, the server would be the person (or voice mail,

computers may have more than one address associated with them. An IP address is a 32-

bit number and is usually represented in dot notation, e.g. 192.168.0.1. Each

section represents one byte of the 32-bit address.

An IP address is like a phone number; a location (residence, business, etc.) can

have one or more phone numbers. To talk to someone at that location, a connection

attempt is initiated (a call is placed and the dialed location's phone rings) by

dialing the phone number for that location. The party at the ringing end of the

phone can then decide whether or not to answer the phone.

Ports

listen on a port. This is why sometimes application or protocol is used

interchangeably with the word port. When a client wants to talk to a server, it must

know where the application is (the IP address/phone number), and which port

(extension) it's listening (answering) on.

Typically, applications have a fixed port so that no matter where they run, the port

is fixed for that type of application. For example, HTTP (Web) uses port 80, and FTP

uses port 21. So when you want to retrieve a Web page, you only need to know the

location of the computer you wish to retrieve it from, as you know HTTP uses port 80.

Port numbers below 1024 are reserved, and should only be used if you're talking to

or implementing a known protocol that has such a port reserved. Most popular

protocols use reserved port numbers.

Sockets

otherwise, no connection (socket) will be established.

Host Names

Host names are "human-readable" names for IP addresses. An example host name is www.

hower.org. Every host name has an equivalent IP address, e.g. www.hower.org =

207.65.96.71.

Host names are used both to make it easier on us humans, and to allow a computer to

change its IP address without causing all of its potential clients (callers) to lose

track of it.

A host name is like a person's name or a business name. A person or business can

change their phone number, but we can still contact them.