Classless and Subnet Address
Extensions (CIDR)
Chapter 10
Introduction
• Five extensions of the IP address scheme,
designed to conserve network prefixes
–
–
–
–
–
Transparent routers
Proxy ARP
Subnet Addressing
Anonymous Point-To-Point Networks
Classless Addressing
Relevant Facts
• In the original IP addressing scheme:
– Each network is assigned a unique network address
– Each host on that network has the network address as a
prefix of the host’s address
• Advantage of this scheme:
– Routers keep one routing entry per network
– Only the network portion of the address is examined
when making routing decisions
Relevant Facts
• Remember original IP addresses
– Class A: 8 bit network id, 24 bit host id
– Class B: 16 bit network id, 16 bit host id
– Class C: 24 bit network id, 8 bit host id
• Sites may modify this scheme as long as:
– All hosts and routers agree to the modified scheme
– Other sites on the Internet can treat addresses as a
network prefix and a host suffix
Minimizing Network Numbers
• Growth has made the original addressing
scheme unfeasible for the future
– Overhead of managing network addresses
– Routing tables are large and exchanging routing
information requires significant effort
– Address space will be exhausted (see p. 148)
• Three ways of sharing one network among
multiple physical networks follows
Transparent Routers
• A router is used to make it look as though
several hosts are connected to a WAN
• It is transparent because other routers and
hosts on the WAN do not know that it exists
• The router is connected to hosts in a local
area network on one side (as a multiplexer),
and to a single host port of the WAN on the
other
H1
Wide Area
Network
H2
T
H3
H4
T is a transparent router connecting multiple
hosts to a WAN. Hosts are assigned addresses
as if they connected directly to the WAN.
Transparent Routers
• The local area network does not have its
own IP prefix
• The router demultiplexes datagrams that
arrive from the WAN and sends them to the
host using a table of addresses
• The router also accepts datagrams from the
hosts and sends them across the WAN to the
destinations
Transparent Routers
• Advantages
– requires fewer network addresses since the
LAN does not need a separate IP prefix
– supports load balancing
• Disadvantages
– works with networks with a large number of
host addresses
• good for class A, not good for class C
– may not provide allservices (ICMP and SNMP)
Proxy ARP
• Applies to networks that use ARP to bind
internet addresses to physical addresses
• Allows one network address to be shared by
two physical networks
• A router which runs proxy ARP answers
ARP requests on each network for hosts on
the other network
• Also called: ARP hack and promiscuous ARP
Main Network
H1
H2
H3
Router running proxy ARP
R
H4
Hidden Network
H5
Proxy ARP
• When H1 needs to talk to H4, it uses ARP
• R captures the ARP request from H1 and
responds with R’s physical address
• H1 sends datagrams destined for H4 to R
• R looks in its routing table to route the
datagram on to H4 on the hidden network
Proxy ARP
• Advantage
– It can be added to a single router without
changing the routing tables in other hosts or
routers on this network
• Disadvantages
– Only works on networks that use ARP
– Spoofing: one machine claims to be another
Subnet Addressing
• Most widely used technique of the 3
• Standardized, required part of IP addressing
• A single site has a single class B address
assigned to it, but has 2 or more networks
• Only local routers know that there are
multiple networks at this site
Network 128.10.1.0
128.10.1.1
128.10.1.2
H1
Rest of the
Internet
H2
R
all traffic to
128.10.0.0
H4
H3
128.10.2.1
Network 128.10.2.0
128.10.2.2
Subnet Addressing
• The address 128.10.0.0 is used for both
networks at the site
• Routers in the internet send to either
network as though it was a single network
• Only R knows that there are two networks
and looks at the third octet to route
– The two networks are called subnets
Subnet Addressing
• Instead of dividing the 32-bit IP address into
(netid, hostid), we use (net portion, local portion)
• The interpretation of the local portion of the
address is left to the site
– The net or internet portion identifies a site
– The local portion identifies a physical network
and a host
Subnet Addressing
• Conceptual 32-bit address in original addressing
with conceptual subnet addressing
– Hierarchical addressing and hierarchical routing
Internet part
Internet part
Local part
Physical
Network
Host
Flexibility in Subnet Address
Assignment
• Sites are allowed flexibility in choice of address
assignment
To the rest of the
R1
Internet
Network 1
R2
Network 3
Network 2
R4
Network 4
R3
R5
Network 5
Flexibility in Subnet Address
Assignment
• See Figure 10.6
– For fixed length subnetting
• When a site has a large number of subnets, the
number of hosts must be small
• When a site has a large number of hosts, the number
of subnets will be small
Variable Length Subnets
• An organization may choose a partition
size for each physical network
– Since the organization may have large and
small networks, this gives flexibility to the site
• Disadvantage:
– Possible address ambiguity
Subnets with Masks
• For subnetting of either kind, a 32-bit
subnet mask specifies the division
– Bits in the mask are set to 1 if machines on the
network treat the corresponding bit in the
address as part of the subnet prefix, 0 if not
– Example:
the mask 11111111 11111111 11111111 00000000
says the first 3 octets identify the network, and the
fourth identifies the host
Subnets with Masks
• Subnet masks do not necessarily have to select
contiguous bits of the address, i.e.:
11111111 11111111 00011000 01000000
… not recommended!
Subnet Mask Representation
• Masks may be represented in dotted decimal
(binary is difficult)
as in 255.255.255.0
• They may be represented as a 3-tuple
{network #, subnet #, host #} where -1 means “all ones”
{-1, -1, 0} is 255.255.255.0
{128.23, -1, 0} is 128.23.255.0
Routing with Subnets
• Hosts connected to networks that are not subnetted
must communicate with hosts on networks that are
subnetted
• Rule: To achieve optimal routing, a machine M must
use subnet routing for an IP network address N, unless
there is a single path P such that P is a shortest path
between M and every physical network that is a subnet
of N.
Routing with Subnets
• Guideline: All subnets of a given network IP
address must be contiguous, the subnet masks
should be uniform across all networks, and all
machines should participate in subnet routing.
Questions
• How does this modify the routing algorithm?
• How are subnet masks assigned?
• How do we broadcast to subnets?
Anonymous Point to Point
Networks
• When a leased line connects two routers,
the line and the two routers are not given
addresses
– No hardware address is needed
– The interface software ignores the next hop
address when sending datagrams
– The connection is known as an unnumbered
network, or anonymous network
128.10.0.0
R1
1
128.211.0.0
leased line
R2
2
128.10.2.250
To reach hosts
on network
128.10.0.0
default
128.211.0.100
Route To
Using Interface #
Deliver Direct
128.211.0.100
Routing Table in R1
1
2
Classless Addressing
• Allows addresses assigned to a single
organization to span multiple classes
• Why adopted?
– The classful scheme did not divide network addresses
into classes equally (<17K class B networks, >2M class
C networks)
– Class C addresses were assigned slowly
– Class B addresses would be exhausted (Running out of
address space ROADS)
Classless Addressing
(Supernetting)
• Consider a medium-sized organization that
joins the Internet
– A class B address is preferred over a class C
– But the organization may be given a block of
256 contiguous class C addresses
– This would also be a useful way to have
Internet Service Providers (ISPs) provide IP
addresses to an organization
• The ISP allocates addresses from the set to subscribers
Supernetting Effects on Routing
• A new problem is created:
– Now routing table is increased incredibly
– Instead of one class B address, we now have
256 class C addresses
• How can the problem be fixed?
– Collapsing a block of contiguous addresses into
a single entry: (network address, count)
• network address is the smallest @ in the block
• count is the number of network @s in the block
Supernetting Effects on Routing
• Example:
– The pair (127.92.61.25, 4) specifies the four
network addresses
•
•
•
•
127.92.61.25
127.92.61.26
127.92.61.27
127.92.61.28
• Routing tables can be smaller
CIDR
• What has just been described is Classless
Inter-Domain Routing (CIDR)
– The name does not indicate that it also involves
addressing
– It is not restricted to Class C addresses
– It does not really use an integer, but requires
that the number of blocks is a power of two,
and this power is identified using a bit mask
CIDR
• Example:
– An organization is assigned a block of 2048
contiguous addresses, beginning at
128.211.168.0
– lowest: 128.211.168.0
10000000 11010011 10100000 00000000
– highest: 128.211.175.255
10000000 11010011 10101111 11111111
CIDR
• CIDR requires 2 things:
– The lowest address in the block
– A 32-bit mask which shows where the division
between prefix and suffix occurs
– 11111111 11111111 11111000 00000000
after the 21st bit in this case
CIDR Notation
• A shorthand way of representing the address
and the mask length is also called slash
notation
• The block of addresses is indicated by the
first address followed by a decimal
indicating the bit position 21
128.211.168.0/21
– See figure 10.11 for CIDR prefixes
CIDR Example
• Work problem in Section 10.21
Summary
• Techniques have been invented to conserve IP
addresses:
– Extend the address space of a single network to include
hosts on an attached local network
– A router answers ARP requests for hosts
– Share one IP network address among several networks
– Let a point-to-point connection be unnumbered
– Allow division between prefix and suffix to occur
anywhere