• 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) - 2 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 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 router of the building is connected to ISDN optic fiber on the interface A (MAC address : 00:11:22:33:44:55). The other interfaces are connected to separated ethernet networks :
  • Interface B : 01:2B:45:56:78:ED
  • Interface C : 01:2B:45:22:88:AF
  • Interface D : 01:2B:45:21:68:75
  • Interface E : 01:2B:45:12:27:33
  • Interface F : 01:2B:45:A4:34:28
  • Interface G : 01:2B:45:11:18:27
• Each Interface B to G is obviously connected to each appartment.

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.
• When you do same thing between 145.134.235.216 and 255.255.255.248 byte after byte what is the result? … do you understand?
• This is the simplest operation to determine as quickly as possible which is the network the IP address belong to and what is the corresponding interface to use.

• 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.