An IPv4 address consists of 32 bits (four octets of 8 bits each), separated into two sections, called the network and the host, by an invisible border. The location of this border is in turn defined by a subnet mask, which is another set of 32 bits that when combined with the IP makes it possible for systems to determine which portion of the IP-address relates to the network, and which portion relates to the host.
Since the introduction of Classless Inter-Domain Routing (CIDR) in 1993, networks are defined using a subnet mask separated from the IP by a slash (eg. 192.0.2.0/24, meaning it contains all the IP-addresses between 192.0.2.0 and 192.0.2.255). The subnet mask (in this case ‘24’) is directly correlated to the amount of remaining bits out of the total of 32, once the network part of the address is accounted for.
For example, the network address 10.0.0.0/8 allows for 32 - 8 = 24 bits (the last three octets) to be used for the host part, resulting in 16 777 216 possible combinations. Similarly, the network 192.0.2.0/24 allows 32-24 = 8 bits to be used for the host part, resulting in only 256 potential individual hosts. The amount of bits available for the host part of the address defines the level of variation possible, or, in other words, the amount of IP-addresses available for use in that particular network.
The practical purpose for the use of CIDR (eg. /24), is to separate the Internet into sections, or networks. This decreases system processing time, by keeping router communication at a need-to-know basis. For example, allocating a large network (eg. /16) to an organization means that other routers only have to keep track of one address for that entire network, since all the IPs contained in that network belong to the same company. As such, all their traffic will go through that route and external communication will be limited to that particular router.
Enter the network route (the address followed by /CIDR) in the field above, and you will be presented with detailed information.