Download Subnets

Chapter
Subnets in TCP/IP Networks
© N. Ganesan, Ph.D.
Chapter Objectives
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Module
Introduction to Subnetting
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
1
140
15
2
Class B Network
140
15
0
0
0
Subnet 1
0
Subnet 2
140
15
Third octet is now used for subnet IDs
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3
0
Subnet 3
Subnet Configuration
Subnet ID
140
140
15
1
15
1
…..
1
0
140
First Host ID
15
1
Last Host ID
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254
Routing of Traffic
140.15.0.0
Routing
Outside world
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140.15.1.0
1
140.15.2.0
2
140.15.3.0
3
Subnets
End of Module
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Module
Subnetting Example
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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Usable
Subnets
(6)
Sub Net
Last Octet
Subnet ID
1
00000000
195.175.25.0
2
00100000
195.175.25.32
3
01000000
195.175.25.64
4
01100000
195.175.25.96
5
10000000
195.175.25.128
6
10100000
195.175.25.160
7
11000000
195.175.25.192
8
11100000
195.175.25.224
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Sample Subnet Division
Subnet 1
Subnet 2
195.175.25.32
195.175.25.64
Router
195.175.25.33
195.175.25.65
.
.
30 hosts per subnet.
.
.
.
.
195.175.25.62
195.175.25.94
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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
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Module
The Routing Process
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 C
0
0
0
0
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
Outside
World
Local Host
195.175.25.40
Router
Subnet Mask: 255.255.255.224
195.175.25.40
195.175.25.67
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Host
195.175.25.34
Computing Subnet ID at Startup
Host ID
Subnet
Mask
ANDing
Result
195
175
25
34
11000011 10101111 00011001 00100010
255
255
255
224
11111111 11111111 11111111 11100000
195
175
25
32
11000011 10101111 00011001 00100000
Yields subnet ID.
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TCP/IP
Properties
of the Host
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Masking of Destination
Address:Case 1
Destinati- 195
nation IP
11000011
Subnet
255
Mask
11111111
ANDing
Result
195
175
25
40
10101111 00011001 00101000
255
255
224
11111111 11111111 11100000
175
25
32
11000011 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- 195
nation IP
11000011
Subnet
255
Mask
11111111
ANDing
Result
195
175
25
67
10101111 00011001 01000011
255
255
224
11111111 11111111 11100000
175
25
64
11000011 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
Subnet at
195.175.25.64
Local Host
195.175.25.40
Router
Subnet Mask: 255.255.255.224
195.175.25.40
(Case 1)
195.175.25.67
(Case 2)
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Host
195.175.25.34
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
255.255.255.254
255.255.255.255
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End of Module
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Module
Subnetting Convention
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
8 Bits
8 Bits
8 Bits
3 Bits
11111111 11111111 11111111 11100000
Total number of masking bits = 27
195.175.25.0/27
Network ID
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Variable Length Subnets
135.41.0.0/16
Subnet
135.41.0.0/17
135.41.128.0.0/17
135.41.128.0./17
1 Network, 32,766 hosts
Subnet
135.41.128.0/21
135.41.248.0/21
15 Networks, 2046 hosts per network
135.41.248.0/24
135.41.255.0/21
8 Networks, 254 hosts per network
Source:
Microsoft White
Paper
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End of Module
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Module
Classless Inter-Domain Routing
(CIDR)
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
Fixed
+ Zeros
Network ID
Variable
Fixed + Variables
Host IDs
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End of Module
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Module
Supernetting
Supernetting and CIDR
Internet Router Entry
8 Network IDs
Network ID
220.78.168.0
220.78.168.0
220.778.168.0
Subnet Mask
255.255.248.0
Network ID
(For supernetting)
Source: Microsoft White
Paper on TCP/IP
.
.
.
220.78.175.0
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End of Module
End of Chapter