Understanding the network.pdf - Back to Home

Figure 2.7. CIDR route summarization and classful
subnetting.
Because CIDR replaces classful addressing and masks, networks needed to be
represented in a format that reflects classless addressing and masking. Under CIDR,
an address space can be any number of addresses that is a power of 2, minus the
network loopback and broadcast address. CIDR address summaries move from the
lowest address space upward. In Figure 2.7, the eight summarized Class C
addresses are represented as the classless network 192.160.0.0 /21
(255.255.248.0). Classless and classful addresses both use 32-bit address masks to
distinguish the network and host portions of the address space. CIDR network
address masks are expressed by indicating the number of binary ones contained in
the address mask instead of the traditional series of four octets. This distinction is
done mainly for semantic purposes, indicating that CIDR is being used for
addressing. From a practical standpoint, the network mask is entered as four octets
when configuring a device with an IP address. Because the CIDR mask
representation is far more elegant than the four octet mask, in the last few years it
has come into common use for describing IP address masks.
Table 2.6 details CIDR bit mask representations in comparison to the traditional
classless four octet mask. Along with a number of possible nodes supported by each
classless address space variation.