sub nets
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Chapter
Subnets in TCP/IP Networks© N. Ganesan, Ph.D.
© N. Ganesan, All rights reserved.
Chapter Objectives
Module
Introduction to Subnetting
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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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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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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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Using Host IDs to Subnet
140 15 0 0
Class B Network140 15 1 0
140 15 2 0
Subnet 1
Subnet 2
Third octet is now used for subnet IDs
140 15 3 0Subnet 3
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Subnet Configuration
140 15 1 0
140 15 1 1
Subnet ID
First Host ID
140 15 1 254…..
Last Host ID
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Routing of Traffic
140.15.0.0 140.15.2.0
140.15.1.0
140.15.3.0
Routing
Subnets
1
2
3Outside world
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End of Module
Module
Subnetting Example
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Subnetting Example
• Consider the case of a class C address 195. 175.25.0 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 0
11100000
Subnet Mask
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Sub Net Last Octet Subnet ID1 00000000 195.175.25.
02 00100000 195.175.25.
323 01000000 195.175.25.
644 01100000 195.175.25.
965 10000000 195.175.25.
1286 10100000 195.175.25.
1607 11000000 195.175.25.
1928 11100000 195.175.25.
224
UsableSubnets
(6)
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Sample Subnet Division
Router
195.175.25.32 195.175.25.64
195.175.25.33...
195.175.25.62
195.175.25.65...
195.175.25.94
30 hosts per subnet.
Subnet 2Subnet 1
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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 255.255.255.224
– 255.255.255.11100000
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End of Module
Module
The Routing Process
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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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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 C0 0 00 1 0
1 0 0
1 1 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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Subnet Masking Example
• Subnet ID: 195.175.25.32• Subnet Mask: 255.255.255.224 • Host address
– 195.175.25.34• Case 1 destination address
– 195.175.25.40• Case 2 destination address
– 195.175.25.67
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Network Scenario
RouterSubnet Mask: 255.255.255.224
Host195.175.25.34
Local Host195.175.25.40
OutsideWorld
195.175.25.40
195.175.25.67
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Computing Subnet ID at Startup
Host ID 195 175 25 3411000011
10101111
00011001
00100010
Subnet Mask
255 255 255 22411111111
11111111
11111111
11100000
ANDingResult
195 175 25 3211000011
10101111
00011001
00100000Yields subnet ID.
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TCP/IPPropertiesof the Host
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Masking of Destination Address:Case 1
Destinati-nation IP
195 175 25 4011000011
10101111
00011001
00101000
Subnet Mask
255 255 255 22411111111
11111111
11111111
11100000
ANDingResult
195 175 25 3211000011
10101111
00011001
00100000
Yields subnet ID to be that of the local subnet.
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Case 1 Forwarding of Data Packets
• The destination host is local• Broadcast for the hardware
address of the local host at IP 195.175.25.40
• Send information to the local host
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Masking of Destination Address:Case 2
Destinati-nation IP
195 175 25 6711000011
10101111
00011001
01000011
Subnet Mask
255 255 255 22411111111
11111111
11111111
11100000
ANDingResult
195 175 25 6411000011
10101111
00011001
01000000
Yields subnet ID to be that of different subnet.
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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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Gateway IP address
specified In TCP/IP
properties.
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Summary of Transmission and Routing of Data
Packets
RouterSubnet Mask: 255.255.255.224
Host195.175.25.34
Local Host195.175.25.40
Subnet at 195.175.25
.64
195.175.25.40(Case 1)
195.175.25.67(Case 2)
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Valid Subnet Masks for Class C Addresses
Subnet Mask Subnets
Hosts Host Total
255.255.255.192
2 62 124
255.255.255.224
6 30 180
255.255.255.240
14 14 196
255.255.255.248
30 6 180
255.255.255.252
62 2 124
255.255.255.254255.255.255.255
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End of Module
Module
Subnetting Convention
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Subnet Convention
• Consider the following Class C example– 195.175.25.0/27
• 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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Subnet Convention Illustrated
11111111
11111111
11111111
11100000
8 Bits 8 Bits 8 Bits 3 Bits
Total number of masking bits = 27
195.175.25.0/27
Network ID
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Variable Length Subnets
Source: Microsoft
White Paper
135.41.0.0/16
135.41.0.0/17 135.41.128.0.0/17
135.41.128.0/21
135.41.128.0./17
135.41.248.0/21
135.41.248.0/24
1 Network, 32,766 hosts
15 Networks, 2046 hosts per network
135.41.255.0/21
Subnet
Subnet
8 Networks, 254 hosts per network
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End of Module
Module
Classless Inter-Domain Routing (CIDR)
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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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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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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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Classless Addressing
Fixed Variable
Fixed + Zeros
Fixed + Variables
Network ID
Host IDs
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End of Module
Module
Supernetting
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Supernetting and CIDR
220.78.168.0
Source: Microsoft White Paper on
TCP/IP
Network ID220.78.168.0
Subnet Mask255.255.248.0
(For supernetting) 220.78.175.0
220.778.168.0
Internet Router Entry
Network ID
.
.
.
8 Network IDs
End of Module
End of Chapter
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