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Subnet & Classless Address
Extensions
Linda Wu
(CMPT 471 • 2003-3)
Content


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
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Motivation
Transparent routers
Proxy ARP
Subnet addressing
Classless addressing
Reference: chapter 10
Notes-4
CMPT 471  2003-3
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Motivation
Problem: network growth will
exhaust IPv4 address space
eventually
 Solution: minimize the number of
addresses used




Notes-4
Avoid assigning netid whenever possible
Share one netid among multiple
networks: transparent router, proxy ARP,
subnet addressing
Arbitrary assignment: classless
addressing
CMPT 471  2003-3
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Transparent Routers



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
Notes-4
A special router T, called transparent router,
connects WAN and LAN
Other hosts and routers on the WAN do not
know T’s existence
The LAN does not have its own netid; the hosts
in the LAN are assigned unused addresses in
the WAN
T sends packets from the WAN to the
appropriate host in the LAN
T accepts packets from the LAN and routes
them across WAN to the destination
CMPT 471  2003-3
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Transparent Routers (cont.)

Advantages



Disadvantages


Notes-4
One netid is shared by multiple networks:
fewer network addresses are required
Support load balancing
Only work with WAN that has a large
address space
Do not provide all the same services as
conventional routers
CMPT 471  2003-3
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Proxy ARP

ARP



Address Resolution Protocol
Maps IP addresses to physical addresses
Proxy

an application that closes a straight path between
2 networks and prevents the crackers from
obtaining internal addresses and details of a
private network
Main network
A
B
C
R
Router running proxy ARP
D
E
Hidden network
Notes-4
CMPT 471  2003-3
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Proxy ARP (cont.)

How proxy ARP works?

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Main network and hidden network: share the
same netid
A router, R, connects these 2 networks
R knows which hosts lie on which physical
networks, and maintains the illusion that only
one network exists
A in the main network sends packets to E in
the hidden network




Notes-4
A broadcasts ARP request for E’s physical addr.
R responses ARP request by sending back its own
physical addr.
A sends the packet destined for E to R
R forwards the packet destined for E over the hidden
network
CMPT 471  2003-3
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Proxy ARP (cont.)

Advantages



Disadvantages

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
Notes-4
One netid is shared by multiple networks
Proxy ARP can be added to a single
router without disturbing other hosts or
routers on the network
The network must use ARP for address
resolution
Cannot be generalized to more complex
network topology
Rely on managers to maintain tables of
machines and addresses manually
CMPT 471  2003-3
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Subnet Addressing
Subnet addressing, subnetting
 A network is divided into several
smaller subnets
 Each subnet has its own subnet
address
 Subnets appear as a single network
to the rest of the internet
 The router attached to the subnets
knows the network is physically
divided into subnets

Notes-4
CMPT 471  2003-3
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Subnet Addressing (cont.)

Subnetting Example
141.14.5.1 141.14.5.2
141.14.5.3
Subnet: 141.14.5.0
141.14.64.1 141.14.64.5 141.14.64.7
Subnet: 141.14.64.0
R1
netid: 141.14.0.0
Class B
Notes-4
141.14.128.1
141.14.128.2
R2
Subnet: 141.14.128.0
CMPT 471  2003-3
To the rest of the internet
10
Subnet Addressing (cont.)

141
Subnetting address
•
14 • 191
netid
•
10
Hierarchy addressing:
hostid
netid
Without subnetting
141
•
netid
14
•
subnetid
•
192
subnetid
192
hostid
hostid
With subnetting
( 405 )
864
Area code
Exchange
-
8902
Connection
Hierarchy in telephone number
Notes-4
CMPT 471  2003-3
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Subnet Addressing (cont.)

Mask


a 32-bit binary number that gives the network
address when bitwise ANDed with an IP address
e.g.
IP address: 123.24.3.1 (class B)
Mask: 11111111 11111111 00000000 00000000
IP & mask = 123.24.0.0 (network address)
mask
IP
address
Notes-4
Bitwise
AND
CMPT 471  2003-3
Network
address
12
Subnet Addressing (cont.)

Default masks
Masks for class A, B, C addresses
 1s: preserve the netid
 0s: set the hostid to 0
 Number of 1s is predetermined: 8/16/24
Notes-4
Class
Binary mask
A
11111111 00000000 00000000 00000000
255.0.0.0
B
11111111 11111111 00000000 00000000
255.255.0.0
C
11111111 11111111 11111111 00000000
255.255.255.0
CMPT 471  2003-3
Dotted-decimal
mask
13
Subnet Addressing (cont.)

Subnet mask



Change some of the leftmost 0s in the
default mask to 1s to make a subnet mask
Preserve netid and subnetid, set hostid to 0
Contiguous subnet mask (recommended)
11111111 11111111 11000000 00000000

Noncontiguous subnet mask
11111111 11111111 00110000 001000000
Default mask: 255.255.0.0
11111111 11111111
00000000
00000000
Subnet mask: 255.255.224.0
11111111 11111111
Notes-4
111 00000 00000000
CMPT 471  2003-3
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Subnet Addressing (cont.)
Subnet mask
255.255.224.0
141.14.72.24 Bitwise
AND
IP address
141.14.64.0
Network address
72  010 01000
224  111 00000
010 00000 (64)
Notes-4
CMPT 471  2003-3
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Subnet Addressing (cont.)

Subnet design example
A company is granted network address
200.16.64.0 (class C). It needs 6 subnets.
Design the subnet.

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Notes-4
# of 1s in the default mask = 24 (class C)
6 subnets < 23: need 3 more 1s in the subnet
mask
Total # of 1s in the subnet mask: 24 + 3 = 27
Total # of 0s in the subnet mask: 8 – 3 = 5
(hostid bits)
Mask is: 11111111 11111111 11111111 11100000,
or, 255.255.255.224
# of hosts per subnet: 25 = 32
CMPT 471  2003-3
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Subnet Addressing (cont.)

Subnet address ranges
1st:
2nd:
3rd :
4th :
5th :
6th :
7th :
8th :
Notes-4
200.16.64.0
200.16.64.32
200.16.64.64
200.16.64.96
200.16.64.128
200.16.64.160
200.16.64.192
200.16.64.224
~
~
~
~
~
~
~
~
200.16.64.31
200.16.64.63
200.16.64.95
200.16.64.127
200.16.64.159
200.16.64.191
200.16.64.223
200.16.64.255
CMPT 471  2003-3
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Subnet Addressing (cont.)

Fixed-length subnetting
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Subnet bits
# of subnets
Hosts per subnet (class B)
0
1 (20)
65534 (216 – 2)
2
2 (22-2)
16382 (214 – 2)
8
254 (28-2)
254 (28-2)
Variable-length subnetting


Notes-4
All 1s or all 0s subnet is not recommended
All 1s and all 0s host addresses are reserved
No single subnetid partition works for all
organizations
An organization may select subnetid partition on
a per-network basis; all hosts and routers
attached to the network must follow the partition
CMPT 471  2003-3
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Subnet Addressing (cont.)

Variable-length subnetting example
A class C site has 5 subnets with host numbers:
60, 60, 60, 30, 30

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2 bits in subnetid? No, only 4 subnets.
3 bits in subnetid? No, at most 32 hosts per
subnets.
62 hosts
First mask (26 1s)
62 hosts
255.255.255.192
62 hosts
router
Notes-4
Second mask (27 1s)
30 hosts
255.255.255.224
30 hosts
CMPT 471  2003-3
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Subnet Addressing (cont.)
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Subnet broadcasting

Subnet broadcast address
hostid is all 1s
 3-tuple form: {netid, subnetid, -1}, “-1”
means “all 1s”.


{netid, -1, -1}
Means “deliver packet to all hosts with
network address netid, even if they are in
separate physical subnets”
 Operationally, such broadcasting make
sense only if routers that interconnect the
subnets agree to propagate the packets
to all subnets

Notes-4
CMPT 471  2003-3
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Classless Addressing
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Also called supernetting
Combine several address blocks to create
a larger address range: supernet
Instead of using a single netid for
multiple subnets, it allows a network’s
addresses to span multiple netids

Notes-4
E.g., an organization that needs 1000 addresses
can be granted 4 class C blocks instead of 1 class
B block
X.Y.32.0 ~ X.Y.32.255
X.Y.33.0 ~ X.Y.33.255
X.Y.34.0 ~ X.Y.34.255
X.Y.35.0 ~ X.Y.35.255
CMPT 471  2003-3
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Classless Addressing (cont.)

Address block assigning

Choose address blocks randomly
The routers outside of the supernet treat
each block separately
 Each router has N entries in its routing table,
N = # of blocks; therefore, increase the size
of the routing table tremendously

Notes-4
CMPT 471  2003-3
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Classless Addressing (cont.)

Choose address blocks based on a set
of rules so that each router has only
one entry in the routing table: required
by CIDR (Classless Inter-Domain
Routing)
# of blocks is a power of 2 (1, 2, 4, 8 …)
 The size of each block is a power of 2
 The blocks are contiguous in the address
space (no gaps between the blocks)
 The size of supernet = (# of blocks) *
(size of each block): a power of 2
 The first address can be evenly divisible
by supernet size

Notes-4
CMPT 471  2003-3
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Classless Addressing (cont.)

Blocks defining in different addressing
schemes
Block: [first address, last address]
 Classful address
one block, default mask is always known
 the first address only can define the block
Subnetting
 the first address in the subblock (subnet) and
subnet mask define the subblock
Supernetting
 the first address of the supernet and supernet
mask define the superblock
 IP address & supernet mask = first address
(network address)



Notes-4
CMPT 471  2003-3
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Classless Addressing (cont.)

Supernet mask

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The reverse of a subnet mask
Has less 1s than the default mask for this class
Subnet mask Divide 1 network into 8 subnets
11111111 11111111 11111111
111
00000
Default mask (class C)
Subnetting
11111111 11111111 11111111
000
00000
Supernetting
Supernet mask Combine 8 networks into 1 supernet
11111111 11111111 11111 000 000 00000
Notes-4
CMPT 471  2003-3
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Classless Addressing (cont.)

Supernet mask examples

A supernet is made out of 16 class C
blocks, what is its supernet mask?
Block #: 16 = 24
Change the last 4 1s in the default mask (class
C) to 0s to get the supernet mask:
11111111 11111111 11111111 00000000
11111111 11111111 11110000 00000000
Notes-4
CMPT 471  2003-3
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Classless Addressing (cont.)

A supernet with mask 255.255.248.0
includes an address 205.16.37.44, what is
the address range?

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Notes-4
First address
205.16.37.44 AND 255.255.248.0
= 205.16.32.0
(11001101 00010000 00100000 00000000)
Mask 11111111 11111111 11111000 00000000,
1s: 21, 0s: 11
Last address: 205.16.39.255 (11001101
00010000 00100111 11111111)
CMPT 471  2003-3
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Classless Addressing (cont.)

Slash notation (CIDR notation):
A.B.C.D/n
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Notes-4
For identifying a CIDR block
A.B.C.D: an IP address
n: # of bits that are shared in every
address in the block, i.e., # of 1s in the
mask
Prefix: common part of the address range
(similar to netid), prefix length = n
Suffix: varying part of the address range
(similar to hostid), suffix length = 32 - n
CMPT 471  2003-3
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Classless Addressing (cont.)

Relationship between mask and prefix
length
/n
/1
Mask
128.0.0.0
/n
/12
Mask
255.240.0.0
/2
192.0.0.0
/16
255.255.0.0 (class B)
/3
/8
224.0.0.0
255.0.0.0 (class A)
/24
/32
255.255.255.0 (class C)
255.255.255.255
Class A: a.b.c.d/8
Class B: a.b.c.d/16
Class C: a.b.c.d/24
Notes-4
CMPT 471  2003-3
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Classless Addressing (cont.)

Subnetting with classless addressing

Increase supernet prefix length (n) to
define the subnet prefix length
Example: an organization is granted the block
130.34.12.64/26. It needs to have 4 subnets.
What is the subnet address and address range
for each subnet?
 Prefix length = 26, suffix length = 6  # of
addresses in the block: 26 = 64
 4 subnets  16 addresses per subnet
 4 subnets  subnet prefix /28 (2 more 1s
in the mask)
Notes-4
CMPT 471  2003-3
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Classless Addressing (cont.)

Subnet address ranges
1st: 130.34.12.64/28 ~ 130.34.12.79/28
 2nd: 130.34.12.80/28 ~ 130.34.12.95/28
 3rd: 130.34.12.96/28 ~ 130.34.12.111/28
 4th: 130.34.12.112/28 ~ 130.34.12.127/28
130.34.12.64/28
130.34.12.80/28
130.34.12.96/28
R1
R2
R3
130.34.12.112/28

Site: 130.34.12.64/26
Notes-4
CMPT 471  2003-3
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Classless Addressing (cont.)

Reserved CIDR blocks
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Notes-4
Private addresses, unroutable addresses
Used with private networks
Never assigned to networks in the global
Internet
Router in the global Internet knows they
are reserved addresses, and can detect it
if a packet destined to the reserved
address accidentally reaches the Internet
CMPT 471  2003-3
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Classless Addressing (cont.)

Notes-4
Reserved CIDR blocks: list
Prefix
First address
Last address
10/8
10.0.0.0
10.255.255.255
172.16/12
172.16.0.0
172.31.255.255
192.168/16
192.168.0.0
192.168.255.255
169.254/16
169.254.0.0
169.254.255.255
CMPT 471  2003-3
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