122 CHAPTER 12: IP ADDRESSING CONFIGURATION
Table 27 describes the address ranges of these five classes. Currently, the first
three classes of IP addresses are used in quantity.
Special Case IP
Addresses
The following IP addresses are for special use, and they cannot be used as host IP
addresses:
■ IP address with an all-zero net ID: Identifies a host on the local network. For
example, IP address 0.0.0.16 indicates the host with a host ID of 16 on the
local network.
■ IP address with an all-zero host ID: Identifies a network.
■ IP address with an all-one host ID: Identifies a directed broadcast address. For
example, a packet with the destination address of 192.168.1.255 will be
broadcasted to all the hosts on the network 192.168.1.0.
Subnetting and Masking Subnetting was developed to address the risk of IP address exhaustion resulting
from fast expansion of the Internet. The idea is to break a network down into
smaller networks called subnets by using some bits of the host-id to create a
subnet-id. To identify the boundary between the host-id and the combination of
net-id and subnet-id, masking is used. (When subnetting is not adopted, a mask
identifies the boundary between the host-id and the host-id.)
Each subnet mask comprises 32 bits related to the corresponding bits in an IP
address. In a subnet mask, the part containing consecutive ones identifies the
combination of net-id and subnet-id whereas the part containing consecutive
zeros identifies the host-id.
Figure 36 shows how a Class B network is subnetted.
Table 27 IP address classes and ranges
Class Address range Description
A 0.0.0.0 to
127.255.255.255
The IP address 0.0.0.0 is used by a host at bootstrap for
temporary communication. This address is never a valid
destination address.
Addresses starting with 127 are reserved for loopback test.
Packets destined to these addresses are processed locally as
input packets rather than sent to the link.
B 128.0.0.0 to
191.255.255.255
--
C 192.0.0.0 to
223.255.255.255
--
D 224.0.0.0 to
239.255.255.255
Multicast address.
E 240.0.0.0 to
255.255.255.255
Reserved for future use except for the broadcast address
255.255.255.255.
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