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Understanding IP Addressing B-3
Class A networks have a small number of possible network
numbers, but a large number of possible host numbers. Conversely,
Class C networks have a small number of possible host numbers,
but a large number of possible network numbers. Thus, the InterNIC
assigns Class A addresses to large organizations that have very
large numbers of IP hosts, while smaller organizations, with fewer
hosts, get Class B or Class C addresses. You can tell the various
classes apart by the value of the first (or high-order) byte. Class A
networks use values from 1 to 127, Class B networks use values
from 128 to 191, and Class C networks use values from 192 to
223. The following table summarizes some of the differences
between Class A, B, and C networks.
Subnets and subnet masks
Often an entire organization is assigned only one IP network
number. If the organization has several IP networks connected
together with IP routers, the network manager can use subnetting to
distinguish between these networks, even though they all use the
same network number. Each physical network becomes a subnet
with a unique subnet number.
Subnet numbers appear within IP addresses, along with network
numbers and host numbers. Since an IP address is always 32 bits
long, using subnet numbers means either the network number or
the host numbers must use fewer bits, in order to leave room for
Class First byte
Number of
networks
possible per
class
Number of
hosts
possible per
network
Format of address
(without subnetting)
Example
A 1-127 127 16,777,214 net.host.host.host 97.3.14.250
B 128-191 16,384 65,534 net.net.host.host 140.100.10.11
C 192-223 2,097,152 254 net.net.net.host 197.204.13.7