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Subnetting

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The subnet mask for the previous example is: 255.255.255. 240. 255.255.255. ... mask with any valid host address on the network will always yield the subnet ... – PowerPoint PPT presentation

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Title: 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?
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