Subnetting

Why Subnet?

• To break the network down into pieces, each of which can be addressed separately.

• Controls network traffic• Reduces broadcasts• Can provide low level security with access lists on the

router• Organization of IP address space

Subnetting a Default Class C Network Address: 200.129.41.0• Default Class C address is divided into network and

host portions as follows:N . N . N . H

• To subnet we “borrow” bits from the host portion of the address (8 bits for Class C)

N . N . N . x x x x x x x x • Borrowing n bits yields 2n – 2 subnets.• Leaving n bits yields 2n – 2 hosts.• For a class C, we can borrow from 2 to 6 bits.• Why not 1 bit? (How many usable subnets?)• Why not 7 bits? (How many usable hosts?)

Subnetting a Default Class C Network Address: 200.129.41.0

• Suppose we need 14 usable subnets, how many bits do we borrow?

• Remember, borrowing n bits give us:• 2n – 2 subnets

• Try borrowing 3 bits (n = 3):• 23 – 2 = 8 – 2

= 6 usable subnets (not enough)• Try borrowing 4 bits

• 24 – 2 = 16 – 2 = 14 usable subnets (enough)

Subnetting a Default Class C Network Address: 200.129.41.0• Write it with the network octet in binary:

200.129.41.0000 0000 break here

• Borrowing 4 bits yields 14 usable subnets• How many usable hosts per subnet?

• Same formula as subnets (2n – 2) • 4 host bits (n = 4)• 24 – 2 = 16 – 2

= 14 usable hosts per subnet

subnet bits host bits


• Examples:• First usable 200.129.41.0001 ^ 0000

subnet address: 200.129.41.16• First usable host 200.129.41.0001 ^ 0001 on the first subnet: 200.129.41.17• Second usable host 200.129.41.0001 ^ 0010

on the first subnet: 200.129.41.18...

• Last usable host 200.129.41.0001 ^ 1110on the first subnet: 200.129.41.30

• Broadcast address 200.129.41.0001 ^ 1111for the first subnet: 200.129.41.31


• Examples:• Second usable 200.129.41.0010 ^ 0000

subnet address: 200.129.41.32• Third usable 200.129.41.0011 ^ 0000

subnet address: 200.129.41.48• Fourth usable 200.129.41.0100 ^ 0000

subnet address: 200.129.41.64...

• Last usable 200.129.41.1110 ^ 0000subnet address: 200.129.41.224

The Subnet Mask: How the Router Determines the Subnet

• The subnet mask (in binary) has:• all ones in the network and subnet portion of the

address• all zeros in the host potion of the address• The subnet mask for the previous example is:

255.255.255. 240255.255.255. 1111^ 0000 (128 + 64 + 32 + 16 =240)

• ANDing this mask with any valid host address on the network will always yield the subnet address for that host, where if both bits are 1 you get a 1 otherwise its a 0.

The Subnet Mask: How the Router Determines the Subnet

• Example (our subnet mask is 255.255.255.240)IP host address: 200.129. 41.23Last octet to binary: 200.129. 41.0001 0111AND subnet mask: 255.255.255.1111 0000

200.129. 41.0001 0000Subnet Address: 200.129. 41.16

So the host address 200.129. 41.23 is on the 200.129.41.16 subnet.

Subnetting a Default Class B Network Address: 132.178.0.0

• Default Class B address is divided into network and host portions as follows:

N . N . H . H• To subnet we “borrow” bits from the host portion

of the address (16 bits for Class B)N . N . x x x x x x x x . x x x x x x x x

• For a class B, we can borrow from 2 to 14 bits.


• Suppose we need 80 usable subnets, how many bits do we borrow?

• Remember, borrowing n bits give us:• 2n – 2 subnets

• Try borrowing 6 bits (n = 6):• 26 – 2 = 64 – 2

= 62 usable subnets (not enough)• Try borrowing 7 bits

• 27 – 2 = 128 – 2 = 126 usable subnets (enough)

Subnetting a Default Class B Network Address: 132.178.0.0• Write it with the network octets in binary:

132.178.0000000 0.00000000 break here

• Borrowing 7 bits yields 126 usable subnets• How many usable hosts per subnet?

• Same formula as subnets (2n – 2) • 9 host bits (n = 9)• 29 – 2 = 512 – 2

= 510 usable hosts per subnet

subnet bits host bits


• Examples:• First usable 132.178.0000001 ^ 0.00000000

subnet address: 132.178.2.0• First usable host 132.178.0000001 ^ 0.00000001

on the first subnet: 132.178.2.1 • Second usable host 132.178.0000001 ^ 0.00000010

on the first subnet: 132.178.2.2 . . .

• Last usable host 132.178.0000001 ^ 1.11111110on the first subnet: 132.178.3.254

• Broadcast address 132.178.0000001 ^ 1.11111111 for the first subnet: 132.178.3.255


• Examples:• Second usable 132.178.0000010 ^ 0.00000000

subnet address: 132.178.4.0• Third usable 132.178.0000011 ^ 0.00000000

subnet address: 132.178.6.0 . . .

• Ninety-first usable 132.178.1011011 ^ 0.00000000subnet address: 132.178.182.0

. . .

• Last usable 132.178.1111110 ^ 0.00000000subnet address: 132.178.252.0


• The subnet mask for this example is:255.255.254.0255.255.1111111 ^ 0.00000000

• ANDing this mask with any valid host address on this network will always yield the subnet address.

• Where if both bits are 1 you get a 1 otherwise its a 0.


• Example:IP host address: 132.178.119.112

Last octets to binary: 132.178.0111011 ^ 1.01110000

AND subnet mask: 255.255.1111111 ^ 0.00000000

132.178.0111011 ^ 0.00000000

Subnet Address: 132.178.118.0

Subnetting

Technology

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