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 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 [Toc] [Back]
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 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 [Toc] [Back]
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.
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 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 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, organization 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.
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
CIDR(7)
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