Small recall on the limitations of IPv4.

- The number of addresses is limited has 232 Adresses minus the addresses of networks and broadcast.

- the tables of routing are full and it is impossible to make configurations more reliable with the only current protocols.

Easy ways :

NAT: To limit the number of IPv4 used on the network, the NAT (Network Adress Translation) was invented it is a method which allows "hidding" a network of the rest of the world the outgoing IP packet on a NAT router carries all the same address what allows has companies not to use that only one address IP known as Publique, the router keeping a table of correspondence to know has which host to send the response of a request. One calls also this kind of method Masquerading but this name tends has to disappear bus being only used by linux (this process initially was indeed related to the Linux Systems) .

But this easy way blocks many applications not being conceived for.

CIDR: much among us knows the classes of addresses of IPv4 which allow a certain distribution of blocks of addresses.

Adress Class Bit of strong weight Adress Bloc Network id Machine id Number of Network Number of Machine/Network


0000 1-126.x.y.z 1-126 x.y.z 126 16 777 214
B 0010 128-191.x.y.z 128-191 y.z 16 384 65 534
C 0110 192-223.x.y.z 192-223 z 2 097 152 254
D 1110 224-239.x.y.z - - - -
E 1111 240-247.x.y.z 240-247      

This way of making is from now on obsolete and is replaced by the CIDR (Classless Inter Domain Routing) which makes it possible to optimize the size of the tables of routings used by the routers of the large world dorsals. The CIDR makes it possible to gather several roads in only one or contrary to dividing networks into sub-networks according to the mask of under network

The rapid exhaustion of the addresses of class B showed that 3 classes of primary addresses were not sufficient class A being too large C too small, the class B being too important for many network but it was best the solution.

a more flexible manner to organize the portions identifying the network and the machine in an address is to use the binary masks, the mask binary function in this way: if a bit is positioned in the mask, the equivalent bit in the address is interpreted as being a bit network

Example :   is under network of the old Class B with a mask of

But      :   is under under customized network that the old notation of classes did not allow normally because a segment of class B is used with a mask of class C

(WELL ON this example addresses of Privée class one uses should not hold account of it)

To use a mask to create networks larger than the natural mask is called suppernetting and the use of a mask instead of the class of address to determine the network of destination is called CIDR (Classless Inter Domain Routing).

Jean Théry

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