2.3 Network layer
2.3.7 Subnetting
When a unit has many hosts and is distributed in a wide geographical range, for the convenience of management, the host numbers inside the unit can be further divided into multiple subnets. Through subnetting, the entire network address can be divided into more small networks.
The division of subnets is an internal behavior of the network. From the outside, this unit has only one network number. Only when the external message enters the range of the unit, the routing device of the unit will select the route according to the subnet number and find the destination host.
As shown in the figure below, it is a class B IP address subnet division, where the subnet mask consists of a series of continuous "1" and a series of continuous "0". "1" corresponds to the network number and subnet number fields, and "0" corresponds to the host number field.
In the figure above, the upper 5 bits of the Host-id of a Class B address are used to divide subnets. The range of Subnet-id is from 00000 to 11111.