4-2
Table 4-1
IP address classes and ranges
Class
Address range
Remarks
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 for 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 addresses.
E
240.0.0.0 to
255.255.255.255
Reserved for future use except for the broadcast address
255.255.255.255.
Special IP Addresses
The following IP addresses are for special use, and they cannot be used as host IP addresses:
z
An 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.
z
An IP address with an all-zero host ID: Identifies a network.
z
An 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 broadcast to all the hosts on the
network 192.168.1.0.
Subnetting and Mask
Subnetting was developed to mitigate IP address exhaustion due to rapid Internet expansion.
Subnetting divides a network into smaller networks called subnets by using some bits of the host ID to
create a subnet ID. A mask identifies the boundary between the host ID and the combined net ID and
subnet ID.
Each subnet mask comprises 32 bits that correspond to the bits in the IP address. In a subnet mask,
consecutive ones represent the combined net ID and subnet ID, and consecutive zeros represent the
host ID.
shows how a Class B network is subnetted.
