1. Subnets in TCP/IP Networks © N. Ganesan, Ph.D.
Chapter
Subnets in TCP/IP Networks
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2. © N. Ganesan, All rights reserved.
Chapter Objectives
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3. Introduction to Subnetting
Module
Introduction to Subnetting

4. © N. Ganesan, All rights reserved.
Subnetting
Division of a network into subnets
For example, division of a Class B address into several Class C addresses
Some of the host IDs are used for creating subnet IDs
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5. © N. Ganesan, All rights reserved.
Need for Subnetting
Classes A and B have a large number of hosts corresponding to each network ID
It may be desirable to subdivide the hosts in Class C subnets
Often, there is a limitation on the number of hosts that could be hosted on a single network segment
The limitation may be imposed by concerns related to the management of hardware
Smaller broadcast domains are more efficient and easy to manage
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6. © N. Ganesan, All rights reserved.
Subnetting Principle
Use parts of the host IDs for subnetting purpose
A subnet mask is used to facilitate the flow of traffic between the different subnets and the outside network (hops)
A hop is the distance a data packet travels form one node to the other
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7. Using Host IDs to Subnet
140 15 1
Subnet 1
Class B Network
140 15 2
140 15
Subnet 2
140 15 3
Subnet 3
Third octet is now used for subnet IDs
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8. © N. Ganesan, All rights reserved.
Subnet Configuration
Subnet ID
140 15 1
…..
140 15 1 1
140 15 1
254
First Host ID
Last Host ID
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Routing of Traffic 1
Routing 2
Outside world 3
Subnets
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10. © N. Ganesan, All rights reserved.
End of Module
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11. Module
Subnetting Example

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Subnetting Example
Consider the case of a class C address assigned to an organization
Subnets can be constructed by allocating part of the higher-order bits of the host ID
Assume that three of the higher-order bits of the host ID are to be reserved for that purpose
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Subnetting Structure
195
175 25
Subnet Mask
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Sub Net
Last Octet
Subnet ID 1 2 3 4 5 6 7 8
Usable
Subnets
(6)
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15. Sample Subnet Division
Router . .
30 hosts per subnet.
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16. Total Number of Subnets and Hosts
All zeros and ones are not used
This has been overcome in the new RFC
Total number of subnets is 6
Number of hosts per subnet is 30
Subnet mask is
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17. © N. Ganesan, All rights reserved.
End of Module
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18. Module
The Routing Process

19. Overview of the Masking Process
IP address and subnet masks are used for the masking operation
The purpose of masking is to identify whether an IP address corresponds to a local host or a remote host
The mathematical technique used is known as the ANDing process
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20. © N. Ganesan, All rights reserved.
ANDing Process
Similar to the AND Boolean operator
Consider A = B and C
A is true only when B and C are true
Otherwise, A is false for all other scenarios
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ANDing Table B C
B AND C 1
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Subnet Masking
AND host IP and subnet mask value at startup to identify network ID
AND destination IP address and subnet mask value determine either of the following:
IP represents local host
IP represents remote host
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23. Subnet Masking Example
Subnet ID:
Subnet Mask:
Host address
Case 1 destination address
Case 2 destination address
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24. © N. Ganesan, All rights reserved.
Network Scenario
Outside
World
Local Host
Router
Subnet Mask:
Host
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25. Computing Subnet ID at Startup
Host ID
195
175 25 34
Subnet
Mask
255
224
ANDing
Result 32
Yields subnet ID.
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TCP/IP
Properties
of the Host
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27. Masking of Destination Address:Case 1
Destinati-nation IP
195
175 25 40
Subnet
Mask
255
224
ANDing
Result 32
Yields subnet ID to be that of the local subnet.
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28. Case 1 Forwarding of Data Packets
The destination host is local
Broadcast for the hardware address of the local host at IP
Send information to the local host
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29. Masking of Destination Address:Case 2
Destinati-nation IP
195
175 25 67
Subnet
Mask
255
224
ANDing
Result 64
Yields subnet ID to be that of different subnet.
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30. Case 2 Forwarding of Data Packets
The destination host is remote
Send information to the gateway
The router at the gateway will route the data packet to the appropriate subnet
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31. © N. Ganesan, All rights reserved.
Gateway IP
address
specified
In TCP/IP
properties.
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32. Summary of Transmission and Routing of Data Packets
Subnet at
Local Host
Router
Subnet Mask:
Host
(Case 1)
(Case 2)
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33. Valid Subnet Masks for Class C Addresses
Subnets
Hosts
Host Total 2 62
124 6 30
180 14
196
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End of Module
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35. Subnetting Convention
Module
Subnetting Convention

36. © N. Ganesan, All rights reserved.
Subnet Convention
Consider the following Class C example
/27
In the above case, the first three octets and the first three higher-order bits of the fourth octet are used in subnet masking
3*8+3 = 27 bits from the beginning of the 32 bit IP address
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37. Subnet Convention Illustrated
8 Bits
8 Bits
8 Bits
3 Bits
Total number of masking bits = 27
/27
Network ID
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38. Variable Length Subnets
/16
Subnet
/17
/17
/17
1 Network, 32,766 hosts
Subnet
/21
/21
15 Networks, 2046 hosts per network
/24
/21
8 Networks, 254 hosts per network
Source: Microsoft White Paper
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39. © N. Ganesan, All rights reserved.
End of Module
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40. Classless Inter-Domain Routing (CIDR)
Module
Classless Inter-Domain Routing (CIDR)

41. Classless Inter-Domain Routing (CIDR)
To avoid the depletion of the class B addresses, it is subnetted and assigned as class C addresses
To avoid the proliferation of network IDs that would complicate entries in the routing tables, they were folded for easing the routing process
The above is known as CIDR
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42. Subnetting of Class B Example
Consider the requirement of 2000 hosts by a company
Allocation of one class B network ID would yield 65,534 hosts
Far more than required
The solution is to subnet a B class address
8 C class network IDs with each network being able to support 254 hosts
The total number of hosts supported is 2,032
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43. © N. Ganesan, All rights reserved.
CIDR
CIDR enables the folding of network IDs
The Internet router tables will need one entry for network ID with the use of a subnet mask for supernetting
Otherwise, the table need to carry 8 entries in the previous example
RIP for IP version 2, OSPF and BGPv2 are protocols that support CIDR
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44. © N. Ganesan, All rights reserved.
Classless Addressing
Fixed Zeros
Network ID
Fixed
Variable
Fixed + Variables
Host IDs
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End of Module
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46. Module
Supernetting

47. Supernetting and CIDR . Internet Router Entry 8 Network IDs Network ID
Subnet Mask
(For supernetting) .
Network ID
Source: Microsoft White Paper on TCP/IP
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48. End of Module
End of Chapter