Day14

1
Day14

• CIDR
• Classless Inter-Domain Routing

2
IP Addresses

• Each computer connected directly to internet must
  have a unique IP address
• Address is made up of 2 parts
• Network Address
• Host Address
• E.g. 10.0.0.1 may be host 1 in the network
  10.0.0.0
• Not all networks are the same size.
• Because of differing needs, some networks can
  hold 2 hosts, some can hold 16,777,216 hosts.
• Therefore we need to be able to specify how much
  of the IP defines is the host address, and how
  much defines the network.

3
Format of IP

• Dotted Decimal
• Each element is called an octet
• There are 4 octets
• Example
• 10.0.0.1
• Binary
• 00001010 00000000 00000000 00000001
• Hex
• 0a000001
• Decimal
• 167,772,161

4
Subnet Masks (netmask)

• A netmask defines how much of the IP address
  represents the network, and how much represents
  the host.
• Example
• My IP address is 123.123.123.5
• Everyone in my network has an IP address
  123.123.123.X
• Network Address is 123.123.123.0
• My host address is .5

5
Netmasks

• A netmask is a 32 bit number just like an IP
  address.
• Valid netmasks have sequential 1s followed by
  0s.
• Example of a valid netmask
• 255.255.255.0
• 11111111.11111111.11111111.00000000
• Example of an Invalid netmask
• 11111111.11111111.11111101.00000000

6
The netmask masks the IP address

• The 1s and 0s in the netmask correspond to the
  network and host portions of the IP address.
• Example
• IP address 10.0.0.1
• 00001010 00000000 0000000 00000001
• Netmask 255.0.0.0
• 11111111 00000000 0000000 00000000
• Thus the network address is
• 10
• And the host address is
• 0.0.1

7
Common Netmasks

• 255.0.0.0 (Class A).
• 255.255.0.0 (Class B).
• 255.255.255.0 (Class C).

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Example

• IP 139.131.1.147 / 255.255.255.0
• What network is this host in?
• 139.131.1.0
• What is the host ID?
• 147

9
Simple netmask example

• Address
• 200.200.200.55
• Netmask
• 255.255.0.0
• 11111111 . 11111111 . 00000000 . 00000000
• This tells us
• 200 . 200 . 200 . 55

Network Address
Host Address
10
It gets worse!

• Unfortunately the host address can start at ANY
  of the 32 bits, not just on the 8 bit boundaries.
• More complex Netmask Example
• 255.255.255.128
• 11111111 . 11111111 . 11111111 . 10000000
• Now our IP address 200.200.200.55 is part of
• Netmask 255.255.255.128
• Lowest Address 200.200.200.0
• Highest Address 200.200.200.127

Network Address
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255.255.255.128 netmask means

• When the netmask was 255.255.255.0 we had 256
  addresses in network
• Once it becomes 255.255.255.128, we now only have
  128 addresses in network.
• If we took a 255.255.255.0 network, and used a
  255.255.255.128 netmask on it
• Our 256 address network, is now divided into 2
  networks
• 200.200.200.0 - 200.200.200.127
• 200.200.200.128 - 200.200.200.255
• They BOTH have the netmask 255.255.255.128

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What are all the valid netmasks?

• 11111111.11111111.11111111.00000000
  255.255.255.0
• 11111111.11111111.11111111.10000000
  255.255.255.128
• 11111111.11111111.11111111.11000000
  255.255.255.192
• 11111111.11111111.11111111.11100000
  255.255.255.224
• 11111111.11111111.11111111.11110000
  255.255.255.240
• 11111111.11111111.11111111.11111000
  255.255.255.248
• 11111111.11111111.11111111.11111100
  255.255.255.252
• 11111111.11111111.11111111.11111110
  255.255.255.254
• 11111111.11111111.11111111.11111111
  255.255.255.255
• Of course, this is only looking at the class C
  networks

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What the netmasks mean

• 255.255.255.0 1 Network 256 addresses
• 255.255.255.128 2 Networks 128 addresses in each
• 255.255.255.192 4 Networks 64 addresses in each
• 255.255.255.224 8 Networks 32 addresses in each
• 255.255.255.240 16 Networks 16 addresses in each
• 255.255.255.248 32 Networks 8 addresses in each
• 255.255.255.252 64 Networks 4 addresses in each
• 255.255.255.254 ??? ???
• 255.255.255.255 This is a single machine.

Whats wrong here?
14
Computing a Netmask

• Given a range of addresses (199.120.197.0 -
  199.120.197.127), you can compute the netmask
• Subtract the lowest number in the range from the
  highest
• 199.120.197.127 - 199.120.197.0 0.0.0.127
• (this means that there are 128 addresses in
  the range -- because 0 counts).
• Subtract the resulting number from
  255.255.255.255
• 255.255.255.255 - 0.0.0.127 255.255.255.128

15
Computing the Address Range

• Given a network (255.255.255.128), you can
  compute the range of addresses
• Subtract the netmask from 255.255.255.255
• 255.255.255.255 - 255.255.255.128 0.0.0.127
• Add 1 to each octet which is gt 0
• 127 1 128 0.0.0.128
• Now you see that there are 128 addresses in this
  range.
• (If your answer were 0.0.128.256, you would know
  that there were 128256 addresses in the range.)

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Another Example

• A Network address is 199.120.197.144
• Its netmask is 255.255.255.240.
• How many IP addresses are in this network?

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Answer

• 199.120.197.144 / 255.255.255.240.
• How many IP addresses are in this network?
• 255.255.255.255 - 255.255.255.240
• 0.0.0.15
• Therefore, we have 16 IP addresses in this
  network
• 199.120.197.144 - 199.120.197.159
• Note That is 16, because 144 is one of them.