Subnet And Mask - H3C S9500 Series Operation Manual

Operation Manual – IP Address
H3C S9500 Series Routing Switches
Network class
D
E
Other
addresses

1.1.2 Subnet and Mask

With rapid development of the Internet, IPv4 addresses are depleting in a few years.
The traditional IP address allocation method wastes IP addresses greatly. In order to
make full use of the available IP addresses, the concept of mask and subnet is
proposed.
A mask is a 32-bit number corresponding to an IP address. The number consists of
contiguous 1s and 0s. In general, a mask is started with consecutive 1s. The mask
divides the IP address into two parts: subnet address and host address. The part of an
IP address that corresponds to the bits 1s in the mask indicates the subnet address and
the other part of IP address indicate the host address. If there is no subnet division,
then its subnet mask is the default value and the length of "1" indicates the net-id length.
Therefore, for IP addresses of classes A, B and C, the default values of corresponding
subnet mask are 255.0.0.0, 255.255.0.0 and 255.255.255.0 respectively.
The mask can be used to divide a Class A network containing more than 16,000,000
hosts or a Class B network containing more than 60,000 hosts into multiple small
networks. Each small network is called a subnet. For example, for the Class B network
address 138.38.0.0, the mask 255.255.224.0 can be used to divide the network into
eight subnets: 138.38.0.0, 138.38.32.0, 138.38.64.0, 138.38.96.0, 138.38.128.0,
138.38.160.0, 138.38.192.0 and 138.38.224.0 (Refer to the following figure). Each
subnet can contain more than 8000 hosts. Subnetting enhances flexibility in network
construction.
Address range
Addresses of class D are multicast
addresses, among which:
224.0.0.0 to
239.255.255.255
240.0.0.0 to
The addresses are reserved for future use.
255.255.255.254
255.255.255.255 is used as a Local Area
255.255.255.255
Network (LAN) broadcast address.
1-3
Chapter 1 IP Address Configuration
Note
IP address 224.0.0.0 is reserved and will
not be allocated. Those from 224.0.0.1 to
224.0.0.255 are reserved for routing
protocols and some protocols that are
used to discover and maintain simple
topologies.
Those from
239.255.255.255 are used for local
multicast management.
Those from
238.255.255.255 are for users.
239.0.0.0 to
224.0.0.255 to