A Better Solution: Eliminate Address Classes
It was clear that as long as there were only three sizes of networks, the allocation efficiency problem could never be properly rectified. The solution was to get rid of the classes completely, in favor of a classless allocation scheme. This system would solve both of the main problems with “classful� addressing: inefficient address space use, and the exponential growth of routing tables.
The idea behind CIDR is to adapt the concept of subnetting a single network to the entire internet. In essence, then, classless addressing means that instead of breaking a particular network into subnets, we can aggregate networks into larger “supernets�. CIDR is sometimes called supernetting for this reason: it applies the principles of subnetting to larger networks. It is this aggregation of networks into supernets that allowed CIDR to resolve the problem of growing Internet routing tables.
Of course, if we are going to apply subnetting concepts to the entire internet, we need to be able to have subnets of different sizes. After all, that’s one of our primary goals in eliminating the classes. So, more accurately, CIDR is an internet-wide application of not regular one-level subnetting, but of Variable Length Subnet Masking (VLSM). Just as VLSM lets us split a network as many times as we want to create subnets, “sub-subnetsâ€? and “sub-sub-subnetsâ€?, CIDR lets us do this with the entire Internet, as many times as needed.
Notação standard
A notação standard para o intervalo de endereços CIDR começa com o endereço de rede (na direita com o número apropriado de bits com valor zero – até 4 octetos para IPv4, e até campos hexadecimais de 8 octetos de 16 bits para IPv6). Isto é seguido por um carácter e comprimento de um prefixo, em bits, definindo o tamanho da rede em questão (o prefixo é, na verdade, o comprimento da máscara de subrede).
Por exemplo:
- 192.168.0.0 /24 representa os 256    endereços IPv4    de 192.168.0.0 até 192.168.0.255 inclusive, com 192.168.0.255 sendo o    endereço de broadcast para a rede.
- 192.168.0.0 /22 representa os 1024    endereços IPv4    de 192.168.0.0 até 192.168.3.255 inclusive, com 192.168.3.255 sendo o    endereço de broadcast para a rede.
- 2002:C0A8::/48 representa os endereços    IPv6 de    2002:C0A8:0:0:0:0:0:0 até 2002:C0A8:0:FFFF:FFFF:FFFF:FFFF:FFFF, inclusive.
Para o IPv4, uma representação alternativa usa o endereço de rede seguido da máscara de subrede, escrito na forma decimal com pontos:
- 192.168.0.0 /24 pode ser escrito    como 192.168.0.0 255.255.255.0
- 192.168.0.0 /22 pode ser escrito    como 192.168.0.0 255.255.252.0
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