The
subnet mask identi?¬?es the network-signi?¬?cant portion of an IP address. The networksigni
?¬?cant portion of an IP address is, simply, the part that identi?¬?es what network the host
device is on (that is, the network address). This is important for the routing operation to be
ef?¬?cient.
Two-Level and Three-Level Addresses
When the IPv4 method of identifying addresses and address classes was developed, a twolevel
address (network and host) seemed suf?¬?cient. Each address class (A, B, and C) had a
default mask associated with it, and because the mask was prede?¬?ned, it was not necessary
to explicitly con?¬?gure the mask.
As the number of network-connected devices grew, it became clear that this was an
inef?¬?cient use of network addresses. To overcome this problem, a third level of addressing,
consisting of subnets, was developed.
A subnet address includes the original classful network portion plus a subnet ?¬?eld. This is
also known as the extended network pre?¬?x. The subnet ?¬?eld and the host ?¬?eld are created
from the original classful host portion. To create a subnet address, you can borrow bits from
the original host ?¬?eld and designate them as the subnet ?¬?eld.
However, subnets cannot work without a way to identify the part of the address that is
network signi?¬?cant and the part that is host signi?¬?cant.
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