Contact:[email protected]
CIDR Block Notations CIDR (Classless Inter-Domain Routing) is a method for creating additional addresses on the Internet. CIDR reduces the burden on Internet routers by aggregating routes so that one IP address represents thousands of addresses that are serviced by a major backbone provider. All packets sent to any of those addresses are sent to the ISP such as UUNET or Level 3. In 1990, there were about 2,000 routes on the Internet. Five years later, there were more than 30,000. Without CIDR, the routers would not have been able to support the increasing number of Internet sites. On a practical level this allows ISPs to easily breakdown the amount of IP addresses they allocate to their business customers. For example the CIDR address 63.80.246.8/29 indicates a block of 8 IP addresses, starting with the 63.80.246.8 and continuing through .15. Using CIDR notation, the larger routers on the Internet to not need to know this, instead they only need to know that 63.80.0.0 is part of UUNET's network, UUNET knows that anything belonging to 63.80.246.0 belongs to Version12, Version12 knows that anything belonging to 63.80.246.8/29 belongs to your company, and your router knows that 63.80.246.12 is your computer. CIDR prefix-length Dotted-Decimal # Individual Addresses # of Classful Networks /13 255.248.0.0 512K 8 Bs or 2048 Cs /14 255.252.0.0 256K 4 Bs or 1024 Cs /15 255.254.0.0 128K 2 Bs or 512 Cs /16 255.255.0.0 64K 1 B or 256 Cs /17 255.255.128.0 32K 128 Cs /18 255.255.192.0 16K 64 Cs /19 255.255.224.0 8K 32 Cs /20 255.255.240.0 4K 16 Cs /21 255.255.248.0 2K 8 Cs /22 255.255.252.0 1K 4 Cs /23 255.255.254.0 512 2 Cs /24 255.255.255.0 256 1 C /25 255.255.255.128 128 1/2 C /26 255.255.255.192 64 1/4 C /27 255.255.255.224 32 1/8 C /28 255.255.255.240 16 1/16 C /29 255.255.255.248 8 1/32 C /30 255.255.255.252 4 1/64 C /31 255.255.255.254 2 1/128 C /32 255.255.255.255 1 1/256 C Back to the Index