chapter subnets in tcp/ip networks © n. ganesan, ph.d

Chapter

Subnets in TCP/IP Networks© N. Ganesan, Ph.D.

© N. Ganesan, All rights reserved.

Chapter Objectives

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


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


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


Using Host IDs to Subnet

140 15 0 0

Class B Network

140 15 1 0

140 15 2 0

Subnet 1

Subnet 2

Third octet is now used for subnet IDs

140 15 3 0

Subnet 3


Subnet Configuration

140 15 1 0

140 15 1 1

Subnet ID

First Host ID

140 15 1 254…..

Last Host ID


Routing of Traffic

140.15.0.0140.15.2.0

140.15.1.0

140.15.3.0

Routing

Subnets

1

2

3Outside world


End of Module

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


Subnetting Structure

195 175 25 0

11100000

Subnet Mask


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)


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


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


End of Module

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


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


ANDing Table

B C B AND C

0 0 0

0 1 0

1 0 0

1 1 1


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


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


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


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.


TCP/IPPropertiesof the Host


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.


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


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.


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


Gateway IP address

specified In TCP/IP

properties.


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)


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


End of Module

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


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


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


End of Module

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


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


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


Classless Addressing

Fixed Variable

Fixed + Zeros

Fixed + Variables

Network ID

Host IDs


End of Module

Module

Supernetting


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

chapter subnets in tcp/ip networks © n. ganesan, ph.d

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