HP 6125G HP 6125G & 6125G/XG Blade Switches Layer 3 - IP Services Conf - Page 28

Special IP addresses, Subnetting and masking, Subnetting a Class B network

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Class D E Address range 224.0.0.0 to 239.255.255.255 240.0.0.0 to 255.255.255.255 Remarks Multicast addresses. 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 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 broadcast to all the hosts on the network 192.168.1.0. Subnetting and masking Subnetting divides a network down into smaller networks called subnets by using some bits of the host ID to create a subnet ID. Masking identifies the boundary between the host ID and the combination of net ID and subnet ID. (When subnetting is not adopted, a mask identifies the boundary between the net ID and the host ID.) Each subnet mask is made up of 32 bits that correspond to the bits in an IP address. In a subnet mask, consecutive ones represent the net ID and subnet ID, and consecutive zeros represent the host ID. Before being subnetted, Class A, B, and C networks use the following default masks (also called natural masks): 255.0.0.0, 255.255.0.0, and 255.255.255.0 respectively. Figure 12 shows how a Class B network is subnetted. Figure 12 Subnetting a Class B network Subnetting increases the number of addresses that cannot be assigned to hosts. After being subnetted, a network can accommodate fewer hosts. For example, a Class B network without subnetting can accommodate 1022 more hosts than the same network subnetted into 512 subnets. • Without subnetting-65,534 hosts (216 - 2). (The two deducted addresses are the broadcast address, which has an all-one host ID, and the network address, which has an all-zero host ID.) • With subnetting-Using the first 9 bits of the host-id for subnetting provides 512 (29) subnets. However, only 7 bits remain available for the host ID. This allows 126 (27 - 2) hosts in each subnet, a total of 64,512 hosts (512 × 126). 20

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20
Class
Address range
Remarks
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 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 broadcast to all the hosts on the network
192.168.1.0.
Subnetting and masking
Subnetting divides a network down into smaller networks called subnets by using some bits of the host ID
to create a subnet ID.
Masking identifies the boundary between the host ID and the combination of net ID and subnet ID.
(When subnetting is not adopted, a mask identifies the boundary between the net ID and the host ID.)
Each subnet mask is made up of 32 bits that correspond to the bits in an IP address. In a subnet mask,
consecutive ones represent the net ID and subnet ID, and consecutive zeros represent the host ID.
Before being subnetted, Class A, B, and C networks use the following default masks (also called natural
masks): 255.0.0.0, 255.255.0.0, and 255.255.255.0 respectively.
Figure 12
shows how a Class B network is subnetted.
Figure 12
Subnetting a Class B network
Subnetting increases the number of addresses that cannot be assigned to hosts. After being subnetted,
a network can accommodate fewer hosts.
For example, a Class B network without subnetting can accommodate 1022 more hosts than the same
network subnetted into 512 subnets.
•
Without subnetting
—65,534 hosts (2
16
– 2). (The two deducted addresses are the broadcast
address, which has an all-one host ID, and the network address, which has an all-zero host ID.)
•
With subnetting
—Using the first 9 bits of the host-id for subnetting provides 512 (2
9
) subnets.
However, only 7 bits remain available for the host ID. This allows 126 (2
7
– 2) hosts in each subnet,
a total of 64,512 hosts (512 × 126).