CIDR(7) 						 Miscellaneous Information Manual						   CIDR(7)

NAME

       CIDR, cidr - Classless Inter-Domain Routing

DESCRIPTION

       Classless  Inter-Domain	Routing,  also known as CIDR, is an Internet addressing architecture designed to solve two problems: the growth in
       the size of the routing tables in the top-level routers and the exhaustion of Class B address space.  To solve these problems, CIDR  relies
       on a new means of distributing the allocation of Internet address space and on a concept known as route aggregation.

       For  an	up-to-date  list  of  Frequently  Asked  Questions (FAQ) about CIDR, retrieve the CIDR FAQ from either of the following locations:
       http://www.rain.net/faqs/cidr.faq.html

       http://www.ibm.net.il/~hank/cidr.html

   Address Space Allocation
       The traditional 32-bit Internet address is divided into a network part and a host part. The size of each part depends on the network  class
       to which the address belongs.  The following table shows the sizes of each part for Class A, Class B, and Class C networks.

       -----------------------------------------------------------------------
       Network	 Size of Network Part (in bits)   Size of Host Part (in bits)
       -----------------------------------------------------------------------
       Class A	 8				  24
       Class B	 16				  16
       Class C	 24				  8
       -----------------------------------------------------------------------

       In  this scenario, each physical network or LAN uses a single network number.  While the idea seems sound, most organizations seldom create
       a single network containing thousands of hosts, choosing instead to divide their networks using routers. For  organizations  with  Class  B
       addresses, this wastes valuable addresses.

       To  cope  with  the  various  network  topologies,  the concept of subnetworks or subnetting emerged.  In a subnet, the network part of the
       address consists of the network part and a portion of the host part. The bitmask convering these two parts is called the subnet	mask.  The
       area  of  the  host  part  that is covered by the subnet mask identifies the subnet. This process allows you to identify individual LANS by
       their subnet number within the larger network number. The only way to communicate between two or more subnets is through a router.

       Currently, routers make routing decisions by extracting the network portion of an IP address and looking it up in their routing table. This
       forces some IP routers to store each network number connected to the Internet in their routing table.

       For many organizations, a Class C network (254 hosts) is too small, whereas a Class B network (65534 hosts) is too large, resulting in poor
       address space utilization.

   Route Aggregation
       The Internet Advisory Board (IAB) and Internet Engineering Task Force (IETF) have decided to eliminate the notion of IP address classes and
       to  direct  routers  to make routing decisions based on a variable-length, contiguous IP address prefix. This is what is meant by classless
       routing. Under this scenario, an Internet Service Provider (ISP) that had previously announced 256  contiguous  Class  C  networks  to  the
       Internet,  now  only  has  to  announce a single prefix, with 16 significant bits, for all these networks. This prefix is referred to as an
       aggregate, and the network is referred to as a supernet.  If the ISP needed to add additional customers to its  network,  it  could  do	so
       without modifying the routing announcements to the rest of the Internet.

       Aggregating networks reduces the number of routers in a network and enables you to make optimum use of bridges and high-speed switches.

EXAMPLES

       This  section  describes  one example of a Class C supernet. If organization A requires 1000 addresses, it might have the following Class C
       networks: 212.221.32.0, 212.221.33.0, 212.221.34.0, and 212.221.35.0.  Using current Class C addressing	specifications,  organization  A's
       network	mask  and  network  numbers  are  as  follows:	/------------24 bits----------- 1111 1111  1111 1111  1111 1111  0000 0000 = mask
       255.255.255.0 1101 0100	1101 1101  0010 0000  0000 0000 = network 212.221.32.0 1101 0100  1101	1101   0010  0001   0000  0000	=  network
       212.221.33.0  1101  0100   1101	1101   0010  0010   0000  0000 = network 212.221.34.0 1101 0100  1101 1101  0010 0011  0000 0000 = network
       212.221.35.0 --------network address-------/  --host--/
					 address Typically, software compares all network address bits that are covered by  the  network  mask	(1
       bits)  to determine the effective network address.  Because the network addresses covered by the 24-bit network mask are different, traffic
       from one network to another requires a router.  In addition, routes to each of the four networks are advertised to the rest of  the  Inter-
       net, and occupy space in the routers' routing tables.

       Under CIDR rules, organization A could shorten their network mask from 24 bits under current rules to 22 bits. The result is a network mask
       of 255.255.252.0, as follows: /-----------22 bits---------- 1111 1111  1111 1111  1111 1100  0000 0000	=  mask  255.255.252.0	1101  0100
       1101  1101   0010  0000	0000 0000 = network 212.221.32.0 1101 0100  1101 1101  0010 0001  0000 0000 = network 212.221.33.0 1101 0100  1101
       1101  0010 0010	0000 0000 = network 212.221.34.0 1101 0100  1101 1101  0010 0011  0000 0000 = network 212.221.35.0 1101  0100	1101  1101
       0010 0011  0000 0000 = network 212.221.35.0 -------network address------/----host----/
				       address	Because  the  network  addresses covered by the 22-bit network mask are the same, traffic from one
       network to another does not require a router. Instead, the software uses Address Resolution Protocol (ARP) to acquire direct connection	to
       the network.

       The  address 212.221.32.0 with the mask 255.255.252.0 identifies all networks belonging to organization A.  Expressed in CIDR format, orga-
       nization A's network address is 212.221.32.0/22.  This effectively aggregates all routes under one network address.  This also  means  that
       only  one  route  is  advertised  to  the  rest	of  the  Internet.  If a router sees traffic addressed to 212.221.33.5 with the netmask of
       255.255.252.0, the traffic is addressed to network 212.221.32.0.

       Using a network mask of 255.255.252.0, organization A can have a single bridged network of 1022 hosts (hosts 0 and 1024	are  reserved  for
       the  broadcast  address).  Using a network mask of 255.255.254.0, organization A can have two bridged networks of 510 hosts (host 0 and 512
       are reserved for the broadcast address).

       These techniques are not currently implemented in all host software, and should be implemented in networks with great  care.  However,  the
       IETF suggests that host software be modified to allow for classless routing.

RELATED INFORMATION

       Commands: netstat(1), ifconfig(8), route(8).

       RFC1517, Applicability Statement for the Implementation of Classless Inter-Domain Routing (CIDR)

       RFC1518, An Architecture for IP Address Allocation with CIDR

       RFC1519, CIDR Address Strategy

       RFC1520, Exchanging Routing Information Across Provider Boundaries in the CIDR Environment delim off

																	   CIDR(7)