1. SUBNET & ROUTING
Sritrusta Sukaridhoto
Lecturer
Pens EEPIS-ITS

2. Netmask / Subnet mask
tutorial

3. IP Addressing

4. IP addressing
•
•
•
•
Introduction to IP addressing
Classes of IP addressing
Why Subnet Masks are necessary?
How to create subnet masks
Subnet & Routing
4

5. Why are IP addresses
written as bits?
• In order for data to pass along the media, it must
first be changed to electrical impulses.
• When a computer receives these electrical
impulses, it recognizes two things: the presence
of voltage on the wire or the absence of voltage
on the wire.
Subnet & Routing
5

6. Subnet & Routing
6

7. What format do IP
addresses use?
Subnet & Routing
7

8. How are IP addresses
expressed in dotted
notation?
Subnet & Routing
8

9. Why are IP addresses
necessary?
• IP addressing makes it possible for data
passing over the network media of the
Internet to find its destination.
• Because each IP address is a 32-bit value,
that means that there are four billion
different IP address possibilities.
• IP addresses are hierarchical addresses
like phone numbers and zip codes.
Subnet & Routing
9

10. How do IP addresses make it
possible for data sent via the
Internet to find its destination?
• It is because each network connected to the
Internet has a unique network number.
• To ensure that each network number on the
Internet will always be unique and unlike that
of any other number, an organization called
the International Network Information
Center, or InterNIC
Subnet & Routing
10

11. How do IP addresses
incorporate network addresses?
• Every IP address has two parts. These are
known as the network number and the host
number.
Subnet & Routing
11

12. What are the different
classes of networks?
• There are three classes of IP
addresses that a company can
receive from the InterNIC. The
InterNIC reserves class "A" IP
addresses for governments
throughout the world, class "B" IP
addresses for medium size
companies, and class "C"
Subnet & Routing
12

13. IP addressing
• Classes of Networks
1 Byte
1 Byte
1 Byte
1 Byte
Class A:
N
H
H
H
Class B:
N
N
H
H
Class C:
N
N
N
H
Network number assigned by NIC
Host number assigned by Systems Administrator
Subnet & Routing
13

14. IP addressing
• IP Address Bit Patterns
1
Bit #
Class A:
0
8
2-8
Network
#
16
9 – 32 (24 bits)
Host #
24
32
Class A address range
1.0.0.0 – 126.0.0.0 (127.0.0.0 is for loopback)
Private Class A address: 10.0.0.0
Number of hosts: 224 -2 = 16,777,214
Subnet & Routing
14

15. IP addressing
• IP Address Bit Patterns
1
Bit #
Class B: 1
8
2
3 - 16
0
17 – 32 (16 bits)
Network # Host #
16
24
32
Class B address range
128.0.0.0 – 191.255.0.0
Private Class B : 172.16.0.0 – 172.31.0.0
Number of hosts: 216 - 2 = 65,534
Subnet & Routing
15

16. IP addressing
• IP Address2 Bit Patterns
1
3
4 - 24
25 – 32 (8 bits)
Bit #
Class C: 1
8
1
16
0
Network # Host #
24
32
Class C address range
192.0.0.0 – 233.255.255.0
Private Class C : 192.168.0.0
Number of hosts: 28 - 2 = 254
Subnet & Routing
16

17. Subnet & Routing
17

18. How many classes of
Networks are there?
• you have learned about three classes
of networks that can be assigned by
the InterNIC.
• In fact, there are five classes of
networks. However, only three of
these are used commercially.
Subnet & Routing
18

19. reserved for multicast
purposes and experimental
purposes?
• The highest number listed was 223.
You may have wondered why the
highest value was only 223 and not
255, since there are 255 possible
values for an octet.
• in IP addresses the values 224
through 255 are not used in the first
octet for networking purposes.
Subnet & Routing
19

20. What IP addresses are
reserved for the Networks?
• By convention, in IP addressing schemes,
any IP address that ends in all binary
zeroes is reserved for the network
address.
• Thus, in a class "A" network, 113.0.0.0
would be the IP address of that network.
Routers use a network's IP address when
forwarding data on the Internet.
Subnet & Routing
20

21. Subnet & Routing
21

22. What IP addresses are
reserved for
broadcasts?
• for the network that is 176.10.0.0, the broadcast
address that would be sent out to all devices on
that network would be 176.10.255.255.
Subnet & Routing
22

23. Who assigns subnet
addresses?
• As with the host number portion of
class "A," class "B," and class "C”
addresses, subnet addresses are
assigned locally.
• Usually this is done by the network
administrator.
Subnet & Routing
23

24. Subnet & Routing
24

25. IP addressing – Subnet
mask
• Subnet Masking - Why?
IP address
131
181
115
252
Netmask
255
255
0
0
Network
Address
131
181
0
0
&
=
10000011 10110101 01110011 11111100
0000000 0000000
11111111 11111111
0
0
Subnet & Routing
25

26. IP addressing Subnetting
• Subnetting
– Is the act of “borrowing” bits from the host
portion to create smaller networks (called
subnetworks)
– Minimum bits that can be borrowed is 2 why?
– Subnetting is used to reduce the number of
broadcast domains
– Communication between these subnetworks is
achieved through a router
Subnet & Routing
26

27. Subnet & Routing
27

28. How are subnet addresses
concealed from outside
networks?
• Subnets are hidden from outside
networks by using a mask.
• These are referred to as subnet
masks.
• The function of a subnet mask is to
tell devices which part of an address
is the network number including the
subnet, and which part is the host.
Subnet & Routing
28

29. What format do subnet
masks use?
• Subnet masks use the same format
as IP addressing.
• In other words, they are thirty two
bits long and divided into four octets.
• Subnet masks have all 1s in the
network and subnetwork portion, and
all 0s in the host portion.
Subnet & Routing
29

30. Subnet & Routing
30

31. Subnet & Routing
31

32. How many bits can be borrowed from
the host number in class "B" and class
"C" networks to create subnets?
• Because there are only two octets in the
host field of a class "B” network, up to
fourteen bits can be borrowed to create
subnetworks.
• A class "C" network has only one octet in
the host field. Therefore, only up to six
bits can be borrowed in class "C” networks
to create subnetworks.
Subnet & Routing
32

33. What happens to the subnet mask
address if only some of the bits in an
octet are borrowed?
• Imagine that you have a class "B” network. This
time however, instead of borrowing all eight bits
of the third octet, only seven bits are borrowed
to create subnetworks.
• Using binary representation, in this example, the
subnet mask would be
11111111.11111111.11111110.00000000.
• Therefore, 255.255.255.0 can no longer be used
as the subnet mask.
Subnet & Routing
33

34. If only seven bits are borrowed in a class
"B" network, what would the subnet mask
be in dotted decimal notation?
• HINT: To convert any eight bit binary number
into a decimal number, total the powers of 2 that
occur in the number.
Subnet & Routing
34

35. What determines how many subnetworks
can be created by borrowing bits from
the host field?
• Can you figure out all of the possible
combinations of 0s and 1s if four bits are
borrowed from the host field to create
subnetworks?
• 16 from 0000 to 1111. However, you know
that 1111 is reserved for broadcast and
0000 means this network.
Subnet & Routing
35

36. How many subnetworks can be
created by borrowing five bits
from the host field?
•
Answer: Thirty-two
subnetworks or 25 =32
subnetworks can be created by
borrowing five bits from the
host field.
Subnet & Routing
36

37. Which numbers in a subnetwork
are reserved for broadcasts?
• In previous section, we used an example of
a class "C" network in which three bits are
borrowed from the host field. You learned
that when three bits are borrowed from
the host octet, up to eight subnetworks
can be created each having up to thirtytwo hosts.
• You also learned that IP addresses ending
in all binary 1s are reserved for
broadcasts. The same is true for
subnetworks. Subnet & Routing
37

38. For IP address 197.15.22.160 on the same
class "C” network. How would this be
expressed in a binary numbering scheme?
• Answer: If 197.15.22.160 is converted to
binary format, it becomes
110001010.00001111.00010110. 10100000.
• The first three bits in the last octet, 101,
indicate that this is the sixth subnetwork.
As before the remaining bits are all binary
0s. This means that the IP
address197.15.22.160 must be one that is
reserved for a subnetwork address.
Subnet & Routing
38

39. Subnet mask
• How do we determine how many bits to
“borrow” for a subnet?
• Determine the number of sub networks
required
• Work from the MOST significant (LHS)
bits of the first octet after the network
number and calculate the number of bits
needed to create the required number of
subnetworks
Subnet & Routing
39

40. Subnet mask
• Example:
– You are given a class B address and you are
required to create 1000 subnetworks.
– By working from the LHS of the (first octet
after the network number) 3rd octet,
calculate the number of bits to equal or
slightly exceed 1000. (ie 2x = > 1000)
– This would equate to 210 or 1024-2 networks
– Hence you will need to borrow 10 bits from the
host portion to create 1000 subnetworks
Subnet & Routing
40

41. Subnet mask
• Example:
– The subnetmask in this instance would
be
10 Bits Borrowed 6 bits left
255.255.255.192
Natural Class B netmask
for hosts
(subnetmask)
11111111
11111111
11111111
11 000000
– How many host per network can you
obtain from this addressing scheme?
Subnet & Routing
41

42. Subnet mask
• How do we determine how many bits to
“borrow” for a subnet given the number of
hosts required?
• Determine the number of hosts required
• Work from the LEAST significant (RHS)
bits of the last octet and calculate the
number of bits needed to create the
required number of subnetworks
Subnet & Routing
42

43. Subnet mask
• Example:
– You are given a class B address and you require
1000 nodes per subnet
– By working from the RHS (last octet) of the
4th octet, calculate the number of bits to
equal or slightly exceed 1000. (ie 2x = > 1000)
– This would equate to 210 or 1024-2 networks
– Hence you will need to borrow 6 bits from the
host portion to create subnetworks with 1000
hosts each
Subnet & Routing
43

44. Subnet mask
• Example:
– The subnetmask in this instance would
be
6 Bits Borrowed 10 bits required
255.255.252.0
Natural Class B netmask
11111111
11111111
(subnetmask)
111111 00
for hosts
00000000
– How many subnetworks per network can
you obtain from this addressing
scheme?
– Note: Do you recognise this address as
Subnet & Routing
44
the student “supernet” address?

45. What about a Supernet?
• A supernet “borrows” bits from the network
portion to create contiguous nodes to form a
“super network”
• For example
– Company A has about 1000 nodes to address. A class B
address would be too big (or may not be available).
Solution Supernetting using 4 contiguous class C
addresses
203.10.112.0
203.10.113.0
203.10.114.0
203.10.115.0
(All netmasked to 255.255.255.0)
Subnet & Routing
45

46. What about a Supernet?
• By allowing the first 2 bits of the third octet
to be “borrowed”, a virtual class B address can
be created.
• A supernet address of 203.10.112.0 –
203.10.115.255 is formed with a subnet mask
of 255.255.252.0.
• The Host portion will be expanded from 8 – 10
bits
• Route summarisation can occur to
203.10.112.0/22
Subnet & Routing
46

47. What about a Supernet?
Network Portion
203.10.112.0
203.10.113.0
203.10.114.0
203.10.115.255
255.255.252.0
1100101
1
1100101
1
1100101
1
1100101
1
1111111
1
00001010
00001010
00001010
00001010
11111111
Host portion
011100
00
011100
01
011100
10
011100
11
111111
00
0000000
0
0000000
0
0000000
0
1111111
1
0000000
0
We have expanded the host portion by 2 bits to 10 bits
Subnet & Routing
47

48. What is a Broadcast
Address
• A broadcast address is used to by a
node to communicate with ALL nodes
in a broadcast domain
• Like the netmask, the broadcast
address is “AND” with the network
address.
• However, the host portion of the
network is identified in a broadcast
address
Subnet & Routing
48

49. What is a Broadcast
Address
• A broadcast address does this by inserting all
“1’s” in the host portion.
• Eg A natural class B broadcast address would look
something like this
– N.N.255.255
• If it is not a classful subnetmask, you can
determine the broadcast address within each
subnet by locating the host portion and setting
them to all 1’s.
Subnet & Routing
49

50. What is a Broadcast
Address
• An example of a Broadcast address
131
181
&
131
181
=
131
181
Host
ID
255
Host
ID
Subnet & Routing
Host
ID
255
Host
ID
50

51. What is a Broadcast Address
• An example of a cross boundary subnet
broadcast address with a mask of
255.255.252.0
131
181
11[2,3,4,5]
x
&
131
181
115
255
=
131
181
SN + H
Host
ID
Network Portion
Network
Address
SN
Host portion
1000001
011100
10110101
1
00
Host
1000001
011100
10110101
Broadcast
Address 1
Subnet & Routingxx
51
1000001
011100
0000000
0
xxxxxxx
x
1111111

52. What is a Broadcast Address
• An example of a cross boundary subnet
broadcast address with a mask of
255.255.252.0
– In this example, IP addresses
• 131.181.112.0 – 131.181.115.255 belong
to the same subnetwork
Network Portion
SN
Host portion
131.181.112.0
(Network)
1000001
011100
10110101
1
00
131.181.113.0
1000001
011100
10110101
1
01
131.181.114.0
1000001
011100
10110101
131.181.115.255 1
10
(Broadacast)
1000001
011100
10110101
Subnet & Routing11
52
1
0000000
0
0000000
0
0000000
0
1111111
1

53. Network Address
VS
Broadcast address
• Remember
• A Network address has all the host
bits set to “0”
• A Broadcast address has all the host
bits set to “1”
• Therefore
– 131.181.112.0 is the network address
– 131.181.115.255 is the broadcast address
Subnet & Routing
53

54. Network Address
VS
Broadcast address
• This is important when you are doing
ifconfig and routing commands
• For example, if a host has an address
131.181.114.10/22
• The ifconfig & route commands would be
– ifconfig eth<x> inet 131.181.114.10 netmask
255.255.252.0 broadcast 131.181.115.255
– route add –net 131.181.112.0 netmask
255.255.252.0 dev eth<x>
Subnet & Routing
54

55. How do you determine
Network and Broadcast
address quickly?
• There are different subnetting exercises
• Given an IP address & mask,
– What is the network/subnetwork address
– What is the network/subnetwork broadcast
address
– What are the assignable address in that
network/subnetwork
– What are all the valid subnet addresses
– How many nodes per subnet
Subnet & Routing
55

56. Algorithm for deriving
Subnet information
• Given an IP address, you will usually be
given a net/subnetmask
• If you are given the mask
– Subtract the mask from 256
– This is known as the multiplier
– The first number in each multiplier value
is the network number
– The broadcast address is the next
multiplier value subtract 1
Subnet & Routing
56

57. Algorithm for deriving
Subnet information
• Eg given the IP address 192.168.0.100
with the subnet mask of
255.255.255.240
Or 192.168.0.100/28
– What is the network number
– What is the broadcast address
– What are the valid IP hosts for the
subnet
Subnet & Routing
57

58. Algorithm for deriving
Subnet information
• Subtract 256 from the netmask
– 256 - 240 = 16
– This is the multiplier ie the networks are in steps of 16
(16,32,48,64,96,112 etc)
• The IP address 192.168.0.100 is in the range of |100 / 16|
which is the 6 th subnetwork
• The network address is
Host portion
16* 6 = 96 (01100000b)
• The Broadcast address is 96 + 16 - 1
=> 192.168.0.111 (01101111b)
– ie (next multiplier – 1)
Subnetwork portion
Subnet & Routing
58

59. Algorithm for deriving
Subnet information
Network Portion
1100000
0
1111111
1
Network Address
1100000
192.168.0.96
Broadcast Address 0
1100000
192.168.0.111
0
IP address
192.168.0.100
Netmask
255.255.255.240
0000000
0
1111111
11111111
1
0000000
10101000
0
0000000
10101000
0
10101000
Subnet & Routing
SN Host portion
0110
0100
1111
0000
0110
0000
0110
1111
59

60. Algorithm for deriving
Subnet information
•
Valid ranges are
– 192.168.0.97 to 192.168.0.110
– Number of allowable hosts 97 to 110 (incl) = 14 or [24]16 - 2 = 14
• Remember you cannot use the first address (network address) and the
last address (broadcast address) in the range
•
The number of allowable networks
– [24]16 - 2 = 14 ( ie 4 bits used. If a class B address with the last bit subnet,
then add another 8 bits to give you 212 –2 allowable subnet)
Subnet & Routing
60

61. Algorithm for deriving
Subnet information
• What if the IP range goes over 2 octets
• Use the same principal
– Remember octets with all 0’s are considered
“boring” and will be assigned the mask of 0
– You will then have to locate the position in the
address with both 1’s and 0’s (interesting byte)
and use the same algorithm
• Similarly all 1’s are also considered boring and will
be given the mask of 255 (eg subnetting the last
byte of a class B address)
Subnet & Routing
61

62. Algorithm for deriving
Subnet information
•
Example
•
•
Netmask expanded : 255.255.252.0
Last byte is “not interesting” hence we set it to “0” for network
and “1” for broadcast
The third byte is “interesting”
256 – 252 = 4 (multiplier)
Networks are in increments of 4 steps
112/4 = 28 (the 28th subnetwork). Since there is no remainder, it
is the beginning of the network address
•
•
•
– QUT students’ “supernet” address
– 131.181.112.0/22
Subnet & Routing
62

63. Algorithm for deriving
Subnet information

•
•
•
•
•
Assignable addresses in this subnetwork
131.181.112.1 – 131.181.115.254
 Remember, the 1st and last addresses
cannot be used (Network and
broadcast)
Network address :
131.181.112.0
Netmask :
255.255.252.0
Broadcast :
112 + 4 – 1 = 115
=>131.181.115.255
Number of valid hosts :
210 –2 = 1024 – 2 = 1022 hosts
Number of subnetworks available for this network
26 – 2 = 64 –2 =62 subnetworks
Subnet & Routing
63

64. How does the router
handle IP addresses and
subnet masks?
• Let's assume that a device on another network
with an IP address of 197.15.22.44 wants to send
data to another device attached to Cisco's
network with an IP address of 131.108.2.2.
• The data is sent out over the Internet until it
reaches the router that is attached to Cisco's
network.
• The router's job is to determine which one of
Cisco's subnetworks the data should be routed to.
Subnet & Routing
64

65. Subnet & Routing
65

66. when the router performs this
”ANDing" operation, the host
portion falls through.
Subnet & Routing
66

67. The router looks at what is left which
is the network number including the
subnetwork.
Subnet & Routing
67

68. The router then looks in its routing table and
tries to match the network number including
the subnet with an interface.
Subnet & Routing
68

69. How does the "Anding” operation
change with different subnet masks?
• Imagine that you have a class "B” network with
the network number 172.16.0.0.
• After assessing the needs of his network, the
network administrator has decided to borrow
eight bits in order to create subnetworks.
• When eight bits are borrowed to create subnets,
the subnet mask is 255.255.255.0.
• Someone outside the network sends data to the
IP address 172.16.2.120.
Subnet & Routing
69

70. Subnet & Routing
70

71. Subnet & Routing
71

72. Exercise
• Exercise
– You are given an IP address for a host
172.168.35.10/20
• What is/are the
– 1. Subnet address?
– 2. Broadcast address?
– 3. The number of useable hosts available
for
this subnet?
– 4. The number of useable subnets available
for this network?
– 5. The assignable address range for this
subnet?
Answers
Subnet & Routing
72

73. Exercise
• Exercise
– Your organisation has been assigned a class B
IP address of 130.10.0.0
– You require about 2000 subnetworks
• Work out the
– 1. Subnet mask required for this subnet
– 2. The network and broadcast addresses for
the first 5 useable subnets
– 3. The number of hosts for each subnet
– 4. The assignable address range of the first
5 useable subnets
Answers
Subnet & Routing
73

74. ROUTING

75. Routing
• Routing describes a process where
packets are forwarded from one
network to another.
• Routing can be performed by devices
such as :
– dedicated routers
– servers with more than one network
interface: multihomed hosts
– switches incorporating a route function.
Subnet & Routing
75

76. Routers
• Routing devices typically have more
than one network interface, each
called a port.
• Routers process datagrams
individually, making routing a
processing-intensive operation.
• Dedicated routers offer better
performance characteristics
compared with multi-homed hosts.
Subnet & Routing
76

77. Routing Tables
• To determining the proper
destination network for datagrams,
routers consult an internal table.
• The table consists of records, one
per line, each representing a known
network.
• Each record includes a set of
associated characteristics such as
netmask
Subnet & Routing
77

78. Building Route Tables.
• Routing table entries can be built by
two methods:
– Static: entries are entered manually by
a network administrator
– Dynamic: entries are entered
dynamically by routing protocols.
Routers learn destination network
addresses by the periodic exchange of
route tables between routing devices.
Routing protocols use IP to deliver this
information.
Subnet & Routing
78

79. Configuring Routers
• Before IP routers can perform the route
function and use routing protocols to
exchange route tables, each interface
(port) must be correctly numbered with a
valid host IP address and netmask.
• The IP address must be selected from
within the range for the particular
network address. Typically local gateways
are located the first address in the valid
host range.
Subnet & Routing
79

80. Routing Protocols
• Routing protocols are used by routers to:
– learn the location of destination networks.
– determine the best route to reach networks.
• Examples of routing protocols include:
– RIP, Routing Information Protocol
– OSPF, Open Shortest Path First
– BGP, Border Gateway Protocol
Subnet & Routing
80

81. Routing Protocols - 2
• Routing protocols differ in:
– the way in which they exchange route
tables
– determine the route to the destination
– the information that is communicated
Subnet & Routing
81

82. Distance Vector
• Distance Vector routing protocols
broadcast the entire route table on a
regular basis. RIP2 typically defaults
at once every 30 seconds.This
creates considerable network
traffic.
• They determine the best route path
on the basis of the least number
number of hops to reach a
Subnet & Routing
82
destination network.

83. Link State
• Link state protocols only broadcast
changes to route information after
an initial entire table has been sent.
• When determining the best path,
other factors such as policies (e.g.
preferred path) and cost ( time
taken, available bandwidth) can
influence the choice when multiple
paths are available.
Subnet & Routing
83

84. IP Routing
• Before a routing device can forward
an IP datagram it must:
– examine the Destination Address in the
datagram
– use the netmask to identify the network
portion of the packet’s destination
address
– find a corresponding network address in
the route table and forward the packet
to the gateway or interface specified
Subnet & Routing
84

85. Route Table Fields
• To forward IP datagrams, the router
uses the following fields of the the
route table:
– Destination
– Network Mask
– Gateway
Subnet & Routing
85

86. Destination
• This field lists the networks which are
known to the router. Addresses may have
been entered by an administrator, or
dynamically learned from the transmissions
of other routers.
• Address entries concerned with routing
between network addresses will be of the
format
{<netid>, 0}
Subnet & Routing
86

87. Netmask
• The netmask field provides the
router with the ability to determine
the network address of packets
being examined.
• A logical AND is performed using the
netmask and the destination address.
This logically removes the host
portion allowing the router to
identify the destination
Subnet & Routing
87

88. Gateway
• The Gateway field lists the IP addresses
of the interface where the datagram
should be sent (forwarded) to reach the
specified Destination.
• This field may contain :
– An IP interface address corresponding to an
adjacent router
– 0.0.0.0
– The address of a interface
Subnet & Routing
88

89. Destination Hop=0.0.0.0
• A Destination of 0.0.0.0 indicates a
directly connected network. Hosts
located on this network can be
reached using the local network
method. If the network is Ethernet,
the ARP protocol is used to find the
physical address of the node.
Subnet & Routing
89