IPv4 addresses are 32-bit numbers that range from 0 to 4,294,967,295 but they are almost never used or seen in that form. Instead, they are usually represented by the familiar "dotted decimal" notation, with four decimal numbers separated by periods, for example, 192.0.2.80 or 126.96.36.199. Each individual number in a dotted decimal ranges in value from 0 to 255, and thus, in dotted decimal notation, IP addresses range from 0.0.0.0 to 255.255.255.255.
Given that there are over 4 billion individual IPv4 addresses, in many circumstances it is impractical to deal with them individually, and instead one deals with ranges (or "blocks") of IPv4 addresses. For example, one may be interested in the range of 256 IPv4 addresses from 192.168.51.0 to 192.168.51.255.
CIDR means'Classless Inter-Domain Routing', it represents the netmask value, as a number between 0 and 32.