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[Veni Markovski]

A Textbook in Internetworking (translated)

DHCP
You wake up after wild drinking. Your first words are "Who am I?", "Where 
am I?". Your neighbour, who did not mix vodka with beer, tells you all your 
data, who you are and where you are. This neighbour behaves like 
DHCP-server. Note that in the Network there may be so-called "fake 
DHCP-servers", e.g. your wife, to the question, "Who am I?", may give you 
false information, "You are an alcoholic". So, obtaining dynamic 
registration is vulnerable, therefore it's recommended that you write your 
data (name, address) on a piece of paper.

Default routing
Go to a pedestrian and ask him/her "could you tell me how to find the 
George W. Bush cemetery?" Most probably you will be sent to a place, which 
contains four letters. That's the so called "default routing", or in other 
words, if you don't know your destination address, the packets are being 
sent there (same like default gateway, or gateway by understanding)

TTL
Imagine you are 5 years old, and want to eat. You go to your Father and say 
"Dad, I want to eat". He is watching TV, and following the routing table, 
sends you to your Mom. You go to her, and ask, "Mom, I want to eat". Your 
Mom talks on the phone with a friend, and following her routing table, 
sends you back to Daddy. So, you walk from your Father to your Mother, just 
because the not so skilled administrators (Mom and Dad), have built 
improper routing table. To prevent such cases, the TTL (Time To Live) was 
created, which in this case means the patience in the kid, until he falls 
into the legs of Mom or Dad, totally tired. The latter is due to send a 
note to the person who has sent the kid to eat; that's the so called ICMP 
packet.

Ping
You've surely have been in situation "you're the fool". You yel "John, 
you're a fool", and in return you hear, "George, you are the fool". That's 
a simple ping. You have just pinged John. Not everyone responds to pings, 
esp. if they are well educated, e.g. Microsoft.com does not react to your 
pings. It's useless to swear them, as we know they can hear us, but there's 
no reaction. In any case, ping is good to see if the host is alive.

Traceroute
Imagine you live on the 9th floor and you want to find out who lives 
between you and Julia, who lives on the 3rd floor. You take a little bomb, 
and by the formula for free fall, you calucalate the explosion to happen on 
the 8th floor, that's TTL=1000 ms. After the bomb explodes, you will see 
the angry face of the neighbour from the 8th floor. Time of reaction 
depends on server average load - is the neighbour busy. If there's no 
answer at all, then your neighbour has icmp reponses forbidden. Then you 
put TTL=2000, etc. However, don't forget, that if Julia lives on the 10th 
floor, that will be "no route to host".