Subnetting

1
Subnetting

• What was that masked address?

2
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

3
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?)

4
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)

5
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
6
Subnetting a Default Class C Network Address
200.129.41.0

• 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 1110
• on the first subnet 200.129.41.30
• Broadcast address 200.129.41.0001 1111
• for the first subnet 200.129.41.31

7
Subnetting a Default Class C Network Address
200.129.41.0

• 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 0000
• subnet address 200.129.41.224

8
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. 240
• 255.255.255. 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.

9
The Subnet Mask How the Router Determines the
Subnet

• Example (our subnet mask is 255.255.255.240)
• IP host address 200.129. 41.23
• Last octet to binary 200.129. 41.0001 0111
• AND subnet mask 255.255.255.1111 0000
• 200.129. 41.0001 0000
• Subnet Address 200.129. 41.16
• So the host address 200.129. 41.23 is on the
  200.129.41.16 subnet.

10
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.

11
Subnetting a Default Class B Network Address
132.178.0.0

• 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)

12
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
13
Subnetting a Default Class B Network Address
132.178.0.0

• 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 1.11111110
• on the first subnet 132.178.3.254
• Broadcast address 132.178.0000001 1.11111111
• for the first subnet 132.178.3.255

14
Subnetting a Default Class B Network Address
132.178.0.0

• 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
  0.00000000
• subnet address 132.178.182.0
• .
• .
• .
• Last usable 132.178.1111110 0.00000000
• subnet address 132.178.252.0

15
Subnetting a Default Class B Network Address
132.178.0.0

• The subnet mask for this example is
• 255.255.254.0
• 255.255.1111111 0.00000000
• ANDing this mask with any valid host address on
  this network will always yield the subnet
  address.

16
Subnetting a Default Class B Network Address
132.178.0.0

• 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
• Which subnet is this. How can you tell?