This is illustrated in Figure 3-29.
Figure 3-29 Classful Pre?¬?x Length
Class C Network Network Network
. . .
Bits to Borrow
Class B Network Network
. . .
Bits to Borrow
Class A Network
. . .
Bits to Borrow
124 Chapter 3: Medium-Sized Routed Network Construction
The subnet address is created by taking address bits from the host-number portion of Class A,
Class B, and Class C addresses. Usually a network administrator assigns the subnet address
locally. Like IP addresses, each subnet address must be unique.
Each time one bit is borrowed from a host ?¬?eld, one less bit remains in the host ?¬?eld that can be
used for host numbers, and the number of host addresses that can be assigned per subnet decreases
by a power of 2.
When you borrow bits from the host ?¬?eld, note the number of additional subnets that are being
created each time one more bit is borrowed. Borrowing two bits creates four possible subnets
(22 = 4). Each time another bit is borrowed from the host ?¬?eld, the number of possible subnets
increases by a power of 2, and the number of individual host addresses decreases by a power of 2.
The following are examples of how many subnets are available, based on the number of host bits
that you borrow:
?– Using 3 bits for the subnet ?¬?eld results in 8 possible subnets (23 = 8).
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