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2-4
Chapter 2
Planning a TCP/IP Network Infrastructure
IP Addresses and Subnet Masks
IP addresses are typically expressed using dotted decimal notation, in which
an
address consists of four integers—often called quads, octets, or
bytes—between 0
and 255, separated by periods. Like an IP address, a subnet mask consists of
32 bits.
In decimal form, the subnet mask appears much like an IP address. In binary
form,
each of the 32 bits has a value of 0 or 1. When you compare a subnet mask
with an
IP address, the address bits that correspond to the 1 bits in the mask are
the network
identifier bits. The address bits that correspond with the 0 bits in the
mask are the
host identifier bits. For example, a typical IP address and subnet mask,
expressed in
the decimal notation used when configuring a TCP/IP computer, appears as
follows:
IP address: 192.168.32.114
Subnet mask: 255.255.255.0
When you convert the address and mask into binary notation, they appear as
follows:
IP address: 11000000 10101000 00100000 01110010
Subnet mask: 11111111 11111111 11111111 00000000
Because the first 24 bits in the subnet mask have the value 1, this
indicates that the
first 24 bits in the IP address make up the network identifier. The final
eight bits in the
mask have the value 0, which means that the final eight bits in the address
are the
host identifier. If the subnet mask value were 255.255.0.0 instead, this
would indicate
that the network identifier and host identifier each consists of 16 bits.
The division
between the 1 and 0 bits can occur almost anywhere in the subnet mask, as
long as
both the network and host identifiers are each at least two bits long.
Using Registered Addresses
To prevent IP address duplication on the Internet, an administrative body
called the IANA
functions as the official IP address registrar. To connect computers
directly to the Internet,
you must obtain a network address from the IANA. A network address is just a
network
identifier. The administrators of the network using that identifier are
responsible for assign-
ing unique host identifiers to the individual computers and other devices on
the network.
By combining the network identifier assigned by the IANA with a unique host
identifier, the
administrators are able to calculate the IP addresses for the computers on
that network.
Off the Record
Although the IANA ultimately assigns all Internet network addresses, network
administrators today do not deal with the address registrar directly.
Instead, they obtain a network
address from an Internet service provider (ISP). The ISP might have obtained
the network address
from a local (LIR), national (NIR), or regional Internet registry (RIR)
(which is assigned pools of
addresses by the IANA directly), but it is also likely that the ISP obtained
the address from its own
service provider. Internet addresses often pass through several layers of
service providers in this
way before they get to the organization that actually uses them.
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