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Latest Articles

TCP/IP Addresses Explained

Arrow Image A break down of TCP/IP addressing



TCP/IP Addresses Explained 22/3/05
=====================================
By n00dles

This txt is a bit more of a tech note, so prob prob not the best for a
total newbie to TCP/IP addressing


Intro
=====
This mini txt describes the structure of TCP/IP addresses and the
meaning of each address component.

TCP/IP uses unique 32-bit binary addresses. These are often called internet or
IP addresses, and are used as universal machine identifiers like a house
number, or a postal address.
IP addresses include both the network and a host on the network. they do not
specify individual machines, but a connection to a network.

Address Classes
---------------

TCP/IP addresses can be one of five classes according to the type of address.
the three main classess are:

Class A allows up to 126 networks, with up to 224 hosts each
Class B allows up to 16 383 networks, each with up to 65 534 hosts
Class C allows up to 221 networks, with up to 254 hosts each

Consequently, each address is broken down into a network identifier (netid)
and the host identifier (hostid).

A bit or bit sequence at the start of each address determines the class of
the address:

Class A is identified by a single \'0\', leaving 31 bits for the address
Class B is identified by the sequence \'10\', leaving 30 bits for the address
Class C is identified by the sequence \'110\', leaving 29 bits for the address

The address format for each of the three classes is a follows:
--------------------------------------------------------------

0 1 2 8 16 24
Class A |0| Netid | Hostid |

Class B |1|0| Netid | Hostid |

Class C |1|1|0| Netid | Hostid |

--------------------------------------------------------------

Dotted decimal Notation
=======================

To present the address in a more understandable form, the 32 bits are broken
down into four 8-bit groups, and are given in decimal.
For example:
192.168.0.1

This Host
---------
Use a hostid consisting entirely of 0s to specify \"this host\", and an internet
address network-id of 0.0.0.0 as \"this network\". You should use this address
only in situations where it can be interpreted unambiguously

Host on this network
--------------------
To identify a host on the local network, use 16 zeros for the first half of the
address, and the next 16 bits to identify the host, for example, 0.0.52.112
would identify an address on the same network.

Limited broadcast
-----------------
You can use limited broadcast addressing to provide a broadcast address for the
whole local network independent of the assigned IP address. A local broadcast
address consists of 32 1s (225.225.225.225). A host can use this address
as a part of a start-up procedure before it establishes its IP address for the
local network

Directed broadcast
-------------------
Use a directed broadcast address to specify broadcast on a targeted network.
It consists f both a valid network-id and a hostid all 1s.
It can therefore be interpreted unambiguously at any point on an internet.
For example:
100.56.225.225

Loopback address
----------------
A packet sent to the network address 127 should not appear on any network.
This address (127.0.0.0)is reserved for loopback and is desigmed for local
testing and inter-process communication.

Subnet addressing
-----------------
Often in a computer network environment, an organizaition may have a single
network address which covers a number of physical networks. The mechanism by
which these are address is known as subnet addressing or subnet routing.

Typically, a site will have a single class-B IP network address, with two or
more physical networks. the local gateway will be connected to the physical
networks and can route the traffic between them.

In this situation, all gateways in the internet behave as if there is a single
physical network. If the the internet tries to address 128.10.0.0 the gateway
determines the subnet address of network A as, typically, 128.10.1.0 and that
of network B as 128.10.2.0.

Summary of address conventions
------------------------------
==========
| all 0s | == The host
==================
| all 0s | host | == Host on this network
==================
| all 1s | == Limited broadcast
==================
| net | all 1s | == Directed broadcast for network
==================
|127 | all 0s | == loopback
==================


The first two formats are only allowed at system startup and are not valid
destination addresses. The limited and directed broadcast formats are not
valid source addresses. The loopback address should never apper on a network

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