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Internet and Transport Protocols : Key Ideas

Internet is a way to send data through successive intermediary links in spite of numerous different network technologies on these links.
Internet is a way to send data world wide, from one interface to another through various intermediary interfaces (routers)
Internet Datagramm are then encapsulated and extracted in/from frames of the intermediary technologies
Each internet internet interface correspond to a unique world wide adress called Internet Protocole Address or IP Adress. Generally for IPv4, it consists in a 4 Bytes number (ex. 157.169.23.34)
to facilitate routing, IP adress are organized hierarchicaly in set of adresses.
Routing table on each router provided a way to know which interface to use to send data to a give IP adress.
In such a hierarchy, every set of IP adresses consists in : a netid part and a hostid part : <netdid>|<hostid>

a subnet is a subset of adresses in this hierarchy : <netid>|<subnetid>|<hostid>
a network mask allows to specify the number of bits of the netid. All the bits corresponding to the netid are equal to 1 and the other to 0. Thus for 167.145.134.32/18 (the means 18 bits to code the netid), the mask is 255.255.192.0.
For each network, size of subnetid + size of hostid = 32 - size of netid. Be careful, <netid>|0..0 and <netid>|1..1 are use respectively to indentify the corresponding network and the broadcast adress (to send data to all the interfaces of the <netid> network at the same time).
So, for each network coded on k bits, a maximum of 2^(32-k-1) adresses are available for hosts.

How many interfaces has your PC ? (use ipconfig or ifconfig commands according to your OS : linus, MacOS, Windows)
What is the IP adress for Wifi ? What is the mask of the corresponding network ? What is the list of IP adresses on this network ?

In a building there is only one connection for all the apartements. The ISDN supplier provides a set of public IP adresses : 145.134.235.32/26.
If I want to provide one public IP adress per appartment, how many appartment can I have in my building ? (answer : 62)
There are 6 appartments in this building. For each appartement I want to provide a subnet. How many bits must I fix for that ? (3)
Can I fix a subnet with 0..0 or 1..1 ? Why ?
What are the different adresses of these networks ? their network mask ?
What maximum of host adresses can I set in each appartment ? (6)

The routing table of the router is then :

(*) Gateway means all the other IP Addresses …

If you use route command (route -print on Windows), you can see the routing table of your computer …. a litle bit more complex, isn't it ?

Comments

using calc application on your PC in programmer mode (see upper left menu), try an AND operator between different hostids of different sunbets and the last byte of the mask …

Ex. : 203 AND 248, gives 200 ; 219 AND 248 gives 216 … do you understand ?

try now a OR oparator between different hostids of different sunbets and the last byte of the mask …

Ex. : 203 OR 248, gives 200 ; 219 AND 248 gives 216 … do you understand ?

Each appartment is then equiped with a main Ethernet plug (on which we can plug a switch to distribute internet connection through ethernet to all the other ethernet plugs of the appartment.

Everything is now defined to design a Network Layout on the floors of the building with conceptdraw , isn't it ?

Because one internet interface can be used to send and receive multiple data simultaneously (from and for multiple software applications on a same interface of a smartphone, a PC, a TV …), transport potocols are just above IP to dispatch these communications.
On each