For example, 198.51.100.0 / 24 is the prefix of the Internet Protocol version 4 network starting at the given address, having 24 bits allocated for the network prefix, and the remaining 8 bits reserved for host addressing. The routing prefix may be expressed in Classless Inter-Domain Routing (CIDR) notation written as the first address of a network, followed by a slash character ( /), and ending with the bit-length of the prefix. The rest field is an identifier for a specific host or network interface. This results in the logical division of an IP address into two fields: the network number or routing prefix and the rest field or host identifier. : 1, 16 The practice of dividing a network into two or more networks is called subnetting.Ĭomputers that belong to the same subnet are addressed with an identical most-significant bit-group in their IP addresses. These networks use the 255.255.255.0 subnet mask, or /24 CIDR notation.Creating a subnet by dividing the host identifierĪ subnetwork or subnet is a logical subdivision of an IP network. The starting host address is 192.0.0.0 and the ending address is 223.255.255.0. These networks use the 255.255.0.0 subnet mask, or /16 CIDR notation.Ĭlass C: Allows 2 8 host addresses on the network. The starting host address is 128.0.0.0 and the ending address is 191.255.0.0. These networks use the 255.0.0.0 subnet mask, or /8 CIDR notation.Ĭlass B: Allows 2 16 host addresses on the network. The starting host address is 0.0.0.0 and the ending address is 127.0.0.0. Specific size details are defined in the following class definitions:Ĭlass A: Allows 2 24 host addresses on the network. Network classes define how many addresses are allowed on the network, with class A being the largest and class C being the smallest. The above table uses Class A, B, and C to define network types.

This means that the last 8 bits of the IP address are reserved for host IP addresses, thus IP addresses 192.168.0.15 through 192.168.0.255 will be allowed as host IP addresses on the network. This means that the first 24 bits of the IP address are reserved for network routing. 192.168.0.15 defines the address prefix, and /24 defines the number of bits reserved for the netmask. To understand what CIDR notation means, take the IP address 192.168.0.15 and the /24 CIDR, for example. CIDR accomplishes the same task as traditional subnet masking. It enables network administrators to group blocks of IP addresses into single routing networks. The following table details how many networks and IP addresses are allowed with each CIDR length and equivalent subnet masks.ĬIDR stands for Classlesss Inter-Domain Routing. CIDR defines which IP addresses, and how many host IP addresses will be allowed on the network. CIDR notation accomplishes the same task as network subnet masking.

Prefix subnet mask table