1-2
Table 1-1 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 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 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 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 IP address with an all-zero host ID: Identifies a network.
z 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.
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 1-2 shows how a Class B network is subnetted.
Figure 1-2 Subnet a Class B network
In the absence of subnetting, some special addresses such as the addresses with the net ID of all zeros
and the addresses with the host ID of all ones, are not assignable to hosts. The same is true for
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