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CIDR was a dramatic break from tradition in
that it completely abandoned the rigid classes of addresses. The original
IPv4 address architecture used an 8-bit network number for Class A
addresses, a 16-bit network number for Class B addresses, and a 24-bit
number for Class C addresses. CIDR replaced these categories with a more
generalized network prefix. This prefix could be of any length rather than
just 8, 16, or 24 bits. This allows CIDR to craft network address spaces
according to the size of a network instead of force-fitting networks into
pre-sized network address spaces.
Each CIDR-compliant network address is advertised with a specific bit
mask. This mask identifies the length of the network prefix. For example,
a network address and SNM of 192.125.61.8 255.255.240.0, is represented
by a 20-bit CIDR network address as: 192.125.61.8/20.
The IP address can be any mathematically valid address regardless of
whether that address was originally part of the Class A, B, or C range!
CIDR-compliant routers look at the number after the /
to determine the network number. Therefore, the former Class C address 192.125.61.8
previously had a network number of 192.125.61
and a host number of 8. As a Class C address, you could provide addresses
for a maximum of 254 hosts within the network. Using CIDR, the
architectural limitations of the 8-bit boundaries between address
components is eliminated. To better understand how this works, it is
necessary to translate the decimal number to binary.
In binary, this network portion of this address is
11000000.0111101.00111101.
The first 20 bits of this example identify the network number. The main
figure demonstrates the split of this
address between network and host numbers.
Notice that the split between the network and host portions of the
address falls in the middle of the third octet. The bits that aren't
allocated to network number are used to identify hosts. Therefore, an IPv4
address with a 20-bit network prefix has 12 bits left for host
identification. Mathematically, this translates to 4,094 usable host
addresses. Because none of the leftmost bits are preset (which previously
established the address class), virtually the entire range of addresses
can be used in a CIDR network. Therefore, a 20-bit network prefix can be
assigned a value that was previously reserved for Class A, B, or C
networks.
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