GCC 12n ネットワークガイド
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IP address class
A Class A IP address uses one byte for the network number and three bytes for the host
number. A Class B address uses two bytes for the network number and two bytes for the
host number. A Class C address uses three bytes for network number and one byte for the
host number. The first few bits of the address determine its class, as follows:
number. A Class B address uses two bytes for the network number and two bytes for the
host number. A Class C address uses three bytes for network number and one byte for the
host number. The first few bits of the address determine its class, as follows:
Class
first bit(s) of first byte
First byte will be
A
0
0 - 127
B
10
128 - 191
C
110
192 - 223
IP addresses are represented as four separate bytes, expressed in decimal, separated by
periods. This is known as dotted decimal notation. The following are examples of dotted
decimal notation:
periods. This is known as dotted decimal notation. The following are examples of dotted
decimal notation:
1.0.3.4
Class A, network 1, host 0.3.4
137.24.9.5 Class B, network 137.24, host 9.5
199.33.4.2 Class C, network 199.33.4, host 2
IP address restrictions
Each IP address consists of the class bits and two separate binary numbers (network num-
ber and host number). There are some restrictions on the values of these numbers. Neither
the network number nor the host number can be designated as zero or all ones. Zero is
reserved; all ones is the broadcast value. Also, there is a special case IP address, known as
the software loopback address. The loopback address is the Class A network number
consisting of all ones (i.e., network number 127). Messages sent to any address at net 127
will come back immediately, without ever going out onto the physical network.
ber and host number). There are some restrictions on the values of these numbers. Neither
the network number nor the host number can be designated as zero or all ones. Zero is
reserved; all ones is the broadcast value. Also, there is a special case IP address, known as
the software loopback address. The loopback address is the Class A network number
consisting of all ones (i.e., network number 127). Messages sent to any address at net 127
will come back immediately, without ever going out onto the physical network.
IP subnet masks
The original IP addressing scheme has been modified to allow a single logical network (the
IP network ) to be divided into multiple physical networks. This is done by redefining
some of the bits from the host number to constitute a subnet number. The designation of a
subnet is optional. However, where subnetting is used, each IP address consists of three
parts:
IP network ) to be divided into multiple physical networks. This is done by redefining
some of the bits from the host number to constitute a subnet number. The designation of a
subnet is optional. However, where subnetting is used, each IP address consists of three
parts:
IP address = <net number><subnet number><host number>
Note: The values zero and all ones are reserved in the subnet number, as they are in the net
number and host number fields.
number and host number fields.