31Class Address range RemarksD 224.0.0.0 to239.255.255.255 Multicast addresses.E 240.0.0.0 to255.255.255.255Reserved for future use except for 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:• 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.• 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 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 of theInternet. The idea is to break a network down into smaller networks called subnets by using some bits ofthe host ID to create a subnet ID. To identify the boundary between the host ID and the combination ofnet ID and subnet ID, masking is used. (When subnetting is not adopted, a mask identifies the boundarybetween the net 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.• The part containing consecutive zeros identifies the host ID.Figure 8 shows how a Class B network is subnetted.Figure 8 Subnetted Class B NetworkWith or without subnetting, some addresses cannot be assigned to hosts because they are reserved forspecial purposes. The address with an all-one host-id is the broadcast address, and the address with theall-zero host-id is the network address.As a result, when designing your network, you should note that subnetting somewhat reduces the numberof hosts that can be accommodated. For example, a Class B network can accommodate 65,534 hosts
