This tutorial is for guys who dun know what happen when two
computers Try to communicate with each others

                                               By E-CriminaL
                      
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This idea has come to my mind when i read a tutorial about how to send
broadcast message.
I found that alot of people dun know what is broadcast , so i decided to
write this tutorial to make these guys understand
what is ARP ,Broadcast,ARP cache,Local routing table and these staff .. so
let`s start
i`ll include my notice between [] ..

1)when your computer or PC want to connect or communicate with another one
in a Local LAN .An ARP [ARP stands for Address Resolution Protocol, this is
used to translate IP protocol to Ethernet Address] request is started, If
the IP address is on the local network, the source host [your computer]
checks its ARP cache [A place in your memory which have some IPs and some
MAC addresses for the local PCs] to see if it already has the hardware
address [MAC address] of the receiving host.Think about it. if your computer
dun know where is the distination place? will it communicate with it? i
guess NO.So the ARP request is important for the connection between
computers in LAN.. If your PC didnt find the MAC address for the destination
PC, a broadcast [with the IP needed] is sent to all local hosts. If the
receiving host finds that the IP address of the source host matches it's own
then it sends a reply to the source host with it's hardware address. When
received by the source host, it's ARP cache is updated to include this info.
If no hosts respond to the broadcast then the request is discarded .. but
wait , the ARP entries in the ARP cache can be stored for 2 min [in case of
dynamic IPs, like Dialup IPs] and stored for the next ReBoot [in case of
static IPs]

2)If your computer want to communicate with a remote computer [i mean not in
the same LAN] .So the source host checks the local routing table for a path
to the destination computer. If one is not found then a broadcast is sent to
the router (gateway). The router replies with its hardware address [MAC
address] and then the packet [this packet is sent from your computer and
include the information you want to send it to the remote computer] is sent
to the router.The router will do the same as what your computer have done
,It checks its cache for the path to the distnation computer. If one is
found then it fowards the packet [your packet]. If not, it sends a broadcast
again and waits for a reply from the host. It may again determine that it is
a remote host and then the process repeats with a broadcast to the next
router and so on ,when the distination host gets the request, it sends an
ICMP echo request and done.. ICMP packets stands for Internet Control
Message Protocol and it is used for error messages for the TCP/IP and for
troubleshooting TCP/IP networks..

3)but some guys dun understand what does it mean router? .. Router or
gateway is a hardware device that fowards packets from one physical network
to another [i said from network to another not from a PC to another in the
same LAN, cause Router dun forward packets from two PCs in one LAN ,i
explained this above]. your PC or your IP uses a routing table to determine
which networks that packets can be forwarded to.. and Routers will only
forward packets to networks that it has been configured to do so. When a
packet is being sent, your computer or more specifically your IP determines
whether or not the IP address is local or remote. If it is local, it forward
the packet. If it is remote, it checks the routing table to determine the
path to the remote host. If there is no entry in the routing table, then the
default gateway [router] is used to do this jop. The router check its
routing table and the process repeats,will the router be able to forward
this packet from the source computer to the distination?if not it will send
a broadcast to the next router and so on.. router to router and router to
router..Each one of these steps is called a \"hop\". If a route to the remote
host is never found then an error message will return. So if you want to see
what i mean .. make a tracert in your MS-DOS or traceroute on your unix
shell and you will see
something like this:

c:\\>tracert 127.0.0.1 [this is localhost.. i mean your IP when u r online or
offline .. so when you ping 127.0.0.1 .. you ping your PC]

Tracing route to 127.0.0.1 over a maximum of 30 hops

This syntax tell you that the tracing process will be for 30 hop only .. so
if you a trace a packet to the distnation computer and this packet passes
through more than 30 router ,the process will be trminated or \"Destination
Unreachable\" error will appear ..

To our main Point \"how to send broadcast message\"

4)IP addresses can be class A, B or C. Class A addresses are for networks
with a huge number of hosts. The first numbers is the NETid like 62.0.0.0
[here in Egypt , for NileOnLine ISP] and the 3 remaining are the HOSTid
,Example .. IPs for \"Link.net\" [a big ISP here in Egypt] will start as
62.1.0.0 to 62.253.x.x [cause 0 and 254 and 255 is reserved] Understood now?
. Class B addresses are used in medium to large networks with the first 2
parts [the NETid],Example .. if egyptnetwork.com give some IPs to cyber like
connect or goal [famous cybers in egypt] the IP in the cyber will be
62[NETid].139[HOSTid for \"link.net\" and NETid for \"egyptnetwork.com\"] and
the remaining 2 are the hostid fot \"egyptnetwork.com\".Class C is for smaller
networks with the first 3 parts are the netid and the last part is the
hostid, example 62[NETid for \"link.net\"].139[The NETid for
\"egyptnetwork.com\" and HOSTid for \"link.net\"].52[HOSTid for
\"egyptnetwork.com\" and NETid for the cyber].252[HOSTid for the cyber] .. i
know this is very complex but relax and read again and again ,You will find
it easy :) And That is for now .. thanx

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About Me; Am E-CriminaL or PortDeviL,18 years old,studing accounting and
production management,computers and networks are my hobbies..

This tutorial is dedicated to Hellkid [very close friend for me] .. we will
form a group for computers , networking and AntiHacking .. So anyone want to
be a member and help us message me at Hacker007egy@hotmail.com Or
Funnest_boy@hotmail.com
Also Want Help in these above sections You Can email me :~)
Wait For Next networking tutorial