An IPv4 address is made of 4 numbers separated by dots as in aaa.bbb.ccc.ddd, each number ranges from 0 to 255.
An IPv4 address range must conform to one of the following forms:
*: the range includes all valid IP addresses,
aaa.*: the range includes the IPv4 addresses which first part is equal to aaa,
aaa.bbb.*: the range includes the IPv4 addresses which first and second parts are respectively equal to aaa and bbb,
aaa.bbb.ccc.*: the range includes the IPv4 addresses which first, second, and third parts are respectively equal to aaa, bbb, and ccc,
aaa.bbb.ccc.ddd-iii.jjj.kkk.lll: the range includes the IPv4 addresses between aaa.bbb.ccc.ddd and iii.jjj.kkk.lll,
aaa.bbb.ccc.ddd/n: the range includes all the IPv4 addresses that belong to the subnet which network prefix or CIDR (Classless Interdomain Routing) notation is aaa.bbb.ccc.ddd/n. An IPv4 address that is included in this range has its first n bits identical to the first n bits of aaa.bbb.ccc.ddd,
aaa.bbb.ccc.ddd-iii.jjj.kkk.lll/n: An IPv4 address that is included in this range is comprised between aaa.bbb.ccc.ddd and iii.jjj.kkk.lll, and has its first n bits identical to the first n bits of aaa.bbb.ccc.ddd and iii.jjj.kkk.lll.
Example D-1. CIDR Notation Example
220.78.168.0/21 represents IPv4 addresses between 220.78.168.0 and 220.78.175.255. Actually, 220.78.168.0 in binary is 11011100 01001110 10101000 00000000. So 220.78.168.0/21 includes all the IPv4 addresses which 21 first bits are 11011100 01001110 10101. Therefore, it represents the range 220.78.168.0-220.78.175.255 (in binary 11011100 01001110 10101000 00000000 - 11011100 01001110 10101111 11111111.)