I don’t remember where I found this, but it’s funny:
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.
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)
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.
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.
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 calculate 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 responses 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”.