Hands-On Microsoft Windows Server 2003

Networking

Chapter Four

Subnetting

2

Objectives

• Understand why subnetting is necessary• Describe how computers work with binary

TCP/IP addresses• Subnet any network• Determine the number of useable hosts on a

subnet• Supernet several smaller networks

3

Reasons to Subnet

• Subnetting– The process of separating a network into several

smaller networks to improve performance

• Subnetting is used because it can– Reduce collisions

– Limit broadcasts

– Control traffic

4

Reducing Collisions

• Collision– When two computers on an Ethernet network

using CSMA/CD as an access method attempt to transmit at the same time

• Subnetting – Reduces number of hosts on each network– Reduces the amount of traffic on the network– With less traffic, collision is reduced

• Collisions that occur on one network do not affect another network

5

Limiting Broadcasts

• Subnetting a network – Creates multiple networks with fewer hosts on

each network

– Fewer hosts results in fewer broadcast messages

• Routers – Packets addressed to IP address

255.255.255.255 are not forwarded

6

Controlling Traffic

• Routers– Allows a greater degree of control over network

traffic

– Most have ability to implement rules about which packets they forward

7

Binary and TCP/IP

• IP addresses– Internally, computer looks at an IP address as a

single group of 32 binary digits

• Decimal numbering– A base-ten numbering system

– Uses the values from 0 to 9 for each digit

8

Counting in Decimal

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Decimal Column Values

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Binary Numbering

• Subnetting– Based on binary

• Binary– Base two numbering system

– Two potential values for each digit: 0 and 1

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Binary Counting

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Binary Column Values

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Conversion Between Binary and Decimal

• To convert a binary octet to a decimal value– Multiply the digit in each column by the value of

each column

– Then determine the sum of those products

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Binary to Decimal Conversion

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Windows Calculator in Scientific Mode

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Binary Subnet Mask• Computer calculates the host ID and network ID

of an IP address while working in binary• Part of the network ID

– Where there is a 1 in the subnet mask• Part of the host ID

– Where there is a 0 in the subnet mask• ANDing

– Binary process used by computers to find the network ID

– An IP address ANDed with a subnet mask gives the network ID

• 1s in a subnet mask must be contiguous

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Calculating Host ID and Network ID

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Invalid Subnet Masks

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Subnetting a Network• To subnet a network

– Take some bits from the host ID and give them to the network ID

• Class B addresses– Normally very large

– Generally need to be subnetted to handle routing between different physical locations

• Using an entire octet for subnetting gives 256 possible subnets

• Traditionally, subnets with all 1s and all 0s are discarded, leaving 254 usable subnets

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Simple Subnetting

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Complex Subnetting

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Subnet Hosts

• 2n-2, where n is the number of host bits – Formula used to calculate the number of subnets

that can be created from a certain number of bits

– Also used to calculate the number of usable hosts on a subnet

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Usable Hosts

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Usable Hosts Formula

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Supernetting

• The opposite of subnetting• Used to create one large network from several

smaller ones• Takes bits from network ID and gives them to

the host ID• All networks being combined for supernetting

must be contiguous• IP addresses from first network to last must be

one single range with no breaks

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Supernetting Two Class C Networks

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Supernetting Four Class C Networks

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Reasons for Supernetting

• Supernetting– Used when a range of IP addresses larger than a

Class C network is required, but a full Class B network is not required

– May also be used to reduce routing complexity

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Summary

• Subnetting– Used to divide a single large network into multiple

smaller networks– Reduces packet collisions– Limits broadcasts– Controls network traffic

• Binary – Base-two numbering system– Only 0 and 1 are valid values

• Computers– Work with IP addresses as 32-digit binary

numbers

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Summary (Continued)• Subnet Mask

– 1 corresponds with bit that is part of network ID– 0 corresponds with bit that is part of host ID

• Subnetting – Takes bits from host ID and uses them as part of

network ID• Number of useable hosts on a network

– Calculated with the formula 2n-2– n is the number of host bits

• Supernetting– Combines multiple smaller networks into a single

larger network– Networks being combined must be contiguous