1-2Table 1-1 IP address classes and rangesClass Address range RemarksA 0.0.0.0 to 127.255.255.255The IP address 0.0.0.0 is usedby a host at bootstrap fortemporary communication. Thisaddress is never a validdestination address.Addresses starting with 127 arereserved for loopback test.Packets destined to theseaddresses are processedlocally as input packets ratherthan 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 addressesE 240.0.0.0 to 255.255.255.255Reserved for future use exceptfor the broadcast address255.255.255.255.Special IP AddressesThe 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 address0.0.0.16 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 packetwith the destination address of 192.168.1.255 will be broadcasted to all the hosts on the network192.168.1.0.Subnetting and MaskingSubnetting was developed to address the risk of IP address exhaustion resulting from fast expansion ofthe Internet. The idea is to break a network down into smaller networks called subnets by using somebits of the host ID to create a subnet ID. To identify the boundary between the host ID and thecombination 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 subnetmask, the part containing consecutive ones identifies the combination of net ID and subnet ID whereasthe part containing consecutive zeros identifies the host ID.Figure 1-2 shows how a Class B network is subnetted.
