Chapter1: Addressing and Subnetting
names to addresses. The internet module maps internet addresses to
local net addresses. It is the task of lower level (i.e., local net
or gateways) procedures to make the mapping from local net addresses
to routes.
Addresses are fixed length of four octets (32 bits). An address
begins with a network number, followed by local address (called the
"rest" field). There are three formats or classes of internet
addresses: in class a, the high order bit is zero, the next 7 bits
are the network, and the last 24 bits are the local address; in
class b, the high order two bits are one-zero, the next 14 bits are
the network and the last 16 bits are the local address; in class c,
the high order three bits are one-one-zero, the next 21 bits are the
network and the last 8 bits are the local address.
IPv4 addresses are expressed in dotted decimal notation. For example, a 32-bit address may look like this
in binary:
To make it easier to read, we take the 32-bit address and group it in blocks of eight bits like this:
Finally, we convert each eight-bit block to decimal and separate the decimal values with periods or
“dots”. The converted IPv4 address, expressed as a dotted decimal address, is:
It is certainly easier to remember that your IP address is 126.136.1.47 instead of remembering a string of
bits such as 01111110100010000000000100101111.
What Is a Network?
When talking about IP addressing, it is important to understand what the word “network” means. A
network is a group of computing devices connected together by some telecommunications medium. It
may be as small as a workgroup in the accounting department or as large as all of the computers in a
large company, such as General Motors. From an addressing perspective, all computers in a network
come under the administration of the same organization. If you want to send information to a computer,
you can identify the computer by its IP address and know that the IP address is assigned to a company.
The IP network can locate the computing resources of the company by locating the network. The network
is identified by a network number.
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