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Table 2.5. Possible Class C Host/Network Classful Subnetting
Breakdown
Mask Size Number of Hosts Number of Networks Decimal Mask
24 254 1 255.255.255.0
25 128 2 255.255.255.128
26 62 4 255.255.255.192
27 30 6 255.255.255.224
28 14 14 255.255.255.240
29 6 30 255.255.255.248
30 2 62 255.255.255.252
Here are some final notes on using classful addressing:
• Only one subnet mask can be used to subnet the entire network space. This
rule applies to Class A, B, and C network address spaces.
• All interfaces connected to a subnet must have host addresses and subnet
masks that reflect the address and subnet mask designated for the network
segment.
• Subnets where the subnet address is all ones will not work, and subnet
addresses of all zeros should be avoided (although some routers will permit
a subnet address of all zeros).
Classless IP Addressing
Address space under IPv4 is tight, and with the Internet growing larger every day,
the usable address space is getting smaller. As it turns out, the classful address
scheme did not work out as well as its designers had thought it would back in 1974.
So, they gave it another try, and came up with supernet addressing and Classless
Interdomain Routing (CIDR).
Supernetting came on the Internet scene in 1993. The idea was to give networks
contiguous groups of Class C addresses that would reflect their actual need instead
of a single large Class B address space that would be mostly wasted. Supernetting
fixed the address shortage, but created routing problems because, although all the
supernet addresses were sequential, they still needed a single classful route for
each address space used. To fix the supernet routing problem, CIDR was developed.
CIDR also arrived in 1993 (when Border Gateway Protocol version 4 was released).
CIDR's fix for the Internet's routing issue was the abandonment of the