cisco networking academy chabot college elec 99.05 subnet masking

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ELEC 99.05ELEC 99.05Subnet Masking

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Logical Address CompositionLogical Address Composition

Though the exact length and format of a logical (layer 3) address differs depending on the protocol, all logical addresses share this basic formula:

NETWORK NUMBER

NETWORK NUMBER HOST NUMBERHOST NUMBER

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Logical Address CompositionLogical Address Composition

IP address may have an additional field, the subnet field, that is used to identify smaller networks within a larger network.

NETWORK NUMBER

NETWORK NUMBER

SUBNET NUMBERSUBNET NUMBER

HOST NUMBER

HOST NUMBER

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Address ClassesAddress Classes

Class A (0 - 127)

Class B (128-191)

Class C (192-223)

Network Host Host Host

Network Network Host Host

Network Network Network Host

1st octet 2nd octet 3rd octet 4th octet

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Address ClassesAddress Classes

Class A

Class B

Class C

85 45 31 158

168 65 114 201

210 144 235 56

Network Host

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The Subnet MaskThe Subnet Mask

• The solution to the IP address shortage was thought to be the subnet mask.

• Formalized in 1985, the subnet mask breaks a single class A, B or C network in to smaller pieces.

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SubnettingSubnetting

What’s happened to the host fields?

Network Network Subnet Host

8 bits 8 bits

8 bits are now used to represent subnets. Only 8 bits remain for possible hosts.

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SubnettingSubnetting

What’s happened to the host fields?

Network Network Subnet Host

8 bits 8 bits

8-bit subnet field = 28 subnets = 256 subnets. 8-bit host field = 28 hosts = 256 hosts. Remember, we started with 65,536 hosts!

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The Subnet MaskThe Subnet Mask

• The Subnet Mask is a 32-bit number.• Its job is to tell routers (and humans)

which bits are network number and which bits are used to represent hosts.

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The Subnet MaskThe Subnet Mask

• The Subnet Mask corresponds to the IP address.

• A “1” bit in the subnet mask means that the corresponding bit in the IP address should be read as a network number

• A “0” bit in the subnet mask means that the corresponding bit in the IP address should be read as a host bit.

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The Subnet MaskThe Subnet Mask

11111111 11111111 00000000 00000000

10101001 11000111 01000101 10001001Address

Mask

Here, the first 16 bits of the mask are set to “1.” Thus, the first 16 bits (2 octets) of the IP address are network number.

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The Subnet MaskThe Subnet Mask

11111111 11111111 00000000 00000000

10101001 11000111 01000101 10001001Address

Mask

Network Network Host Host

The mask shows that the first two octets refer to the network number.

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The Subnet MaskThe Subnet Mask

11111111 11111111 00000000 00000000

10101001 11000111 01000101 10001001Address

Mask

Network Network Host Host

The mask shows that the first two octets refer to the network number.

255 255 0 0

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The Subnet MaskThe Subnet Mask

11111111 11111111 11111111 00000000

10101001 11000111 01000101 10001001Address

Mask

Network Network Network Host

Here, the first 24 bits are set to “1” in the subnet mask. Thus, the first 24 bits (3 octets) of the IP address are network number.

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The Subnet MaskThe Subnet Mask

11111111 11111111 11111111 00000000

10101001 11000111 01000101 10001001Address

Mask

Network Network Network Host

Here, the first 24 bits are set to “1” in the subnet mask. Thus, the first 24 bits (3 octets) of the IP address are network number.

255 255 255 0

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The Subnet MaskThe Subnet Mask

11111111 11111111 11111111 11100000

10101001 11000111 01000101 10001001Address

Mask

Network Network Network Host

Here, the first 27 bits of the subnet mask are set to “1.” Thus, the first 27 bits of the IP address refer to network number.

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The Subnet MaskThe Subnet Mask

11111111 11111111 11111111 11100000

10101001 11000111 01000101 10001001Address

Mask

Network Network Network Host

Here, the first 27 bits of the subnet mask are set to “1.” Thus, the first 27 bits of the IP address refer to network number.

255 255 255 224

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The Default Subnet MasksThe Default Subnet Masks

Class A or /8

Class B or /16

Class C or /24

255 0 0 0

255 255 0 0

255 255 255 0

If any additional bits in the subnet mask are set to “1,” then subnets have been created.

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The Subnet MaskThe Subnet Mask

255 255 255 0

207 21 54 0Address

Mask

Network Network Network Host

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The Subnet MaskThe Subnet Mask

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 00000000

255 255 255 0

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Class C : 1-bit Mask (/25)Class C : 1-bit Mask (/25)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 10000000

255 255 255 128

Masks can be written different ways. When the phrase “1-bit” mask is used, that means 1 more bit than the default. This example can also be called a 25-bit mask, or /25 (there are 25 network bits).

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Class C: 2-bit Mask (/26)Class C: 2-bit Mask (/26)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 11000000

255 255 255 192

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Class C: 3-bit Mask (/27)Class C: 3-bit Mask (/27)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 11100000

255 255 255 224

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Class C: 4-bit Mask (/28)Class C: 4-bit Mask (/28)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 11110000

255 255 255 240

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Class C: 5-bit Mask (/29)Class C: 5-bit Mask (/29)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 11111000

255 255 255 248

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Class C: 6-bit Mask (/30)Class C: 6-bit Mask (/30)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 11111100

255 255 255 252

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Class C: 7-bit Mask (/31)Class C: 7-bit Mask (/31)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 11111110

255 255 255 254

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Class C: 8-bit Mask (/32)Class C: 8-bit Mask (/32)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network

11111111 11111111 11111111 11111111

255 255 255 255

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Class C : 1-bit Mask (/25)Class C : 1-bit Mask (/25)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 10000000

255 255 255 128

Because this is a class C, all nodes on this network will share the first three octets. Those numbers are not an issue.

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Last Octet 1-bit Mask (/25)Last Octet 1-bit Mask (/25)

00000000

0Address

Mask

Host

10000000

128

How many subnet possibilities with 1 bit?

21 = 2 different possibilities

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Last Octet 1-bit Mask (/25)Last Octet 1-bit Mask (/25)

000000000 =

128 =

255 =

The last octet can be any value from 0 - 255.

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

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Last Octet 1-bit Mask (/25)Last Octet 1-bit Mask (/25)

000000000 =

128 =

255 =

How many hosts can be on each subnet?

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

27 = 128 different possibilities MINUS TWO.

7 bits

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Why “Minus Two”?Why “Minus Two”?• Two special host addresses are “reserved”:

– the address of the subnet itself(all zeros)

– the broadcast address for the subnet(all ones)

000000000 =

128 =

255 =

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

Subnet addr.

Broadcast addr.

Subnet addr.

Broadcast addr.

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Network #, Broadcast AddressNetwork #, Broadcast Address

000000000 =

All zeros in the HOST portion of the address is reserved for network (or subnet) number.

All ones is reserved for the network (or subnet) BROADCAST address.

000000011 =

Decimal Binary

0010010036 =

01111111127 =

10000000128 =

11111111255 =

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Last OctetLast Octet

000000000 =

128 =

255 =

So, which subnet is this host on?

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

207.21.54.35

255.255.255.128Subnet #0

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Last OctetLast Octet

000000000 =

128 =

255 =

Are these two hosts on the same subnet?

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

207.21.54.115

255.255.255.128

No.207.21.54.129

255.255.255.128

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Last OctetLast Octet

000000000 =

128 =

255 =

Unfortunately, older TCP/IP software cannot handle subnets with all zeros in the subnet field.

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

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Last OctetLast Octet

000000000 =

128 =

255 =

More bad news, older TCP/IP software cannot handle subnets with all ones in the subnet field.

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

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Last OctetLast Octet

000000000 =

128 =

255 =

For now, using subnets that have all zeros or all ones in the subnet field should be considered illegal. (It’s not really, but….)

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

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Last OctetLast Octet

000000000 =

128 =

255 =

Bottom line, a 1-bit mask results in no usable addresses.

01111111127 =

10000000

11111111

Subnet #0

Subnet #1

Decimal Binary

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Class C : 2-bit Mask (/26)Class C : 2-bit Mask (/26)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 11000000

255 255 255 192

Because this is a class C, all nodes on this network will share the first three octets. Those numbers are not an issue.

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Class C : 2-bit Mask (/26)Class C : 2-bit Mask (/26)

00000000

0Address

Mask

Host

11000000

192

How many subnet possibilities with 2 bits?

22 = 4 different possibilities:

00 01 10 11

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Last OctetLast Octet

000000000 =

64 =

127 =

0011111163 =

01000000

01111111

Subnet #0

Subnet #1

Decimal Binary

128 =

191 =

10000000

10111111

192 =

255 =

11000000

11111111

Subnet #2

Subnet #3

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Last OctetLast Octet

000000000 =

64 =

128 =

How many hosts can be on each subnet?

0011111163 =

01000000

01111111

Subnet #0

Subnet #1

Decimal Binary

26 = 64 different possibilities MINUS TWO.

6 bits

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Which subnets can be used?Which subnets can be used?

000000000 =

64 =

127 =

0011111163 =

01000000

01111111

Subnet #0

Subnet #1

Decimal Binary

128 =

191 =

10000000

10111111

192 =

255 =

11000000

11111111

Subnet #2

Subnet #3

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Putting it togetherPutting it togetherAssuming the mask: 255.255.255.192, which addresses are legal for hosts?

1) 207.21.54.35

2) 207.21.54.63

3) 207.21.54.65

4) 207.21.54.190

5) 207.21.54.195

000000000 =

64 =

127 =

0011111163 =

01000000

01111111

Subnet #0

Subnet #1

Decimal Binary

128 =

191 =

10000000

10111111

192 =

255 =

11000000

11111111

Subnet #2

Subnet #3

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Class C : 3-bit Mask (/27)Class C : 3-bit Mask (/27)

11001111 00010101 00110110 00000000

207 21 54 0Address

Mask

Network Network Network Host

11111111 11111111 11111111 11100000

255 255 255 224

Because this is a class C, all nodes on this network will share the first three octets. Those numbers are not an issue.

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Class C : 3-bit Mask (/27)Class C : 3-bit Mask (/27)

00000000

0Address

Mask

Host

11100000

224

How many subnet possibilities with 3 bits?

23 = 8 different possibilities:

000 001 010 011 100 101 110 111

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Last OctetLast Octet

000000000 =

32 =

63 =

0001111131 =

00100000

00111111

Decimal Binary

64 =

95 =

01000000

01011111

96 =

127 =

01100000

01111111

10000000128 =

160 =

191 =

10011111159 =

10100000

10111111

Decimal Binary

192 =

223 =

11000000

11011111

224 =

255 =

11100000

11111111

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Last OctetLast Octet

000000000 =

32 =

64 =

How many hosts can be on each subnet?

0001111131 =

00100000

00111111

Subnet #0

Subnet #1

Decimal Binary

25 = 32 different possibilities MINUS TWO.

5 bits

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Which subnets can be used?Which subnets can be used?

000000000 =

32 =

63 =

0001111131 =

00100000

00111111

Decimal Binary

64 =

95 =

01000000

01011111

96 =

127 =

01100000

01111111

10000000128 =

160 =

191 =

10011111159 =

10100000

10111111

Decimal Binary

192 =

223 =

11000000

11011111

224 =

255 =

11100000

11111111