CIDR man page on DigitalUNIX

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CIDR(7)								       CIDR(7)

       CIDR, cidr - Classless Inter-Domain Routing

       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 exhaus‐
       tion 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:

   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	Size of Host Part (in bits)
		 (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 bit‐
       mask 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	 rout‐
       ing.  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 addi‐
       tional 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.

       This section describes one example of a Class C supernet. If  organiza‐
       tion  A	requires  1000	addresses, it might have the following Class C
       networks:,,,  and
       Using  current Class C addressing specifications, organization A's net‐
       work mask and network numbers are as follows:

       /------------24 bits-----------\ 1111 1111  1111 1111  1111 1111	  0000
       0000  = mask 1101 0100  1101 1101	0010 0000  0000 0000 =
       network 1101 0100  1101 1101  0010 0001  0000 0000 =  net‐
       work 1101 0100  1101 1101	0010 0010  0000 0000 = network 1101 0100  1101 1101   0010	 0011	0000  0000  =  network \--------network address-------/  \--host--/

       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 adver‐
       tised to the rest of the Internet, 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, as follows:

       /-----------22  bits----------\	1111  1111  1111 1111  1111 1100  0000
       0000 = mask 1101 0100  1101 1101  0010 0000  0000 0000  =
       network 1101 0100	1101 1101  0010 0001  0000 0000 = net‐
       work 1101 0100  1101 1101  0010 0010  0000 0000 =  network  1101  0100	  1101	1101   0010  0011  0000 0000 = network 1101 0100  1101 1101   0010	 0011	0000  0000  =  network \-------network address------/\----host----/

       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 with the  mask  identifies  all
       networks	 belonging to organization A.  Expressed in CIDR format, orga‐
       nization A's network  address  is   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	with  the  netmask  of, the traffic is addressed to network

       Using a network mask of, 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, 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

       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


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