IP Address Configuration 81
A mask is a 32-bit number corresponding to an IP address. The number consists of
1s and 0s. Principally, these 1s and 0s can be combined randomly. However, the
first consecutive bits are set to 1s when designing the mask. The mask divides the
IP address into two parts: subnet address and host address. The bits 1s in the
address and the mask indicate the subnet address and the other bits 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 8 subnets: 138.38.0.0, 202.38.32.0, 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
Figure 22). Each subnet can contain more than 8000 hosts.
Figure 22 Subnet Division of IP Address
Configuring IP Address Configure an IP address for a VLAN interface in one of three ways:
â– Using the IP address configuration command
â– Allocated by BOOTP server
â– Allocated by DHCP server
These three methods are mutually exclusive and a new configuration will replace
the current IP address. For example, if you apply for an IP address using the
ip
address bootp-alloc
command, the address allocated by BOOTP shall replace
the currently-configured IP address.
This section introduces how to configure an IP address with the IP address
configuration command. The other two methods are described in subsequent
chapters.
10001010, 00100110, 000 00000, 00000000
ClassB
138.38.0.0
Subnet mask
255.255.224.0
11111111, 11111111, 111 00000, 00000000
11111111, 11111111, 000 00000, 00000000
Standard
mask
255.255.0.0
Subnet address:
000 Subnet address: 138.38. 0. 0
001 Subnet address: 138.38. 32. 0
010 Subnet address: 138.38. 64. 0
011 Subnet address: 138.38. 96. 0
100 Subnet address: 138.38.128. 0
101 Subnet address: 138.38.160. 0
110 Subnet address: 138.38.192. 0
111 Subnet address: 138.38.224. 0
Subnet
number
Host
number
Subnet address:
10001010, 00100110, 000 00000, 00000000
ClassB
138.38.0.0
Subnet mask
255.255.224.0
11111111, 11111111, 111 00000, 00000000
11111111, 11111111, 000 00000, 00000000
Standard
mask
255.255.0.0
Subnet address:
000 Subnet address: 138.38. 0. 0
001 Subnet address: 138.38. 32. 0
010 Subnet address: 138.38. 64. 0
011 Subnet address: 138.38. 96. 0
100 Subnet address: 138.38.128. 0
101 Subnet address: 138.38.160. 0
110 Subnet address: 138.38.192. 0
111 Subnet address: 138.38.224. 0
Subnet
number
Host
number
Subnet address:
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