IP Fragmentation and Reassembly

1
IP Fragmentation and Reassembly

• Example
• 4000 byte datagram
• MTU 1500 bytes

1480 bytes in data field
offset 1480/8
2
IP Addressing

• Bit
• One digit, a zero or one (0, 1)
• Byte
• Eight bits, any combination of zero and ones
• Also called an octet

3
IP datagram

• At a destination host
• reconstruct payload
• pass to the transport layer
• An incomplete datagram is discarded
• How to handle this problem?

4
IP Addressing Review Binary

• Each combination of eight bits (in an octet) can
  be converted to a decimal number
• Each bit position has a decimal value assigned to
  it

5
Binary

• When Converting
• If the bit is zero the value of the bit is zero
  
• If the bit is one the bit has the value of the
  decimal conversion
• Total the value of the decimal conversion of all
  the octets bits to determine the decimal
  conversion number of the octet

6
Number of Addresses Calculation

• Calculating number of addresses from a specific
  number of bits
• One bit Two Addresses
• 0
• 1
• Two bits Four Addresses
• 00
• 01
• 10
• 11

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Number of Addresses Calculation

• Four bits 16 Addresses, etc.
• Eight bits 256 Addresses
• 0000 0000
• 0000 0001
• 0000 0010
• 0000 0011
• 0000 0100
• 0000 0101
• 1111 1111

8
IP Addressing

• Binary Number
• 00000000
• 00000001
• 00000010
• 00000011
• 00000100
• 00000101
• 00000110
• 00000111
• 11111111

• Decimal Number
• 0
• 1
• 2
• 3
• 4
• 5
• 6
• 7
• 255

9
IP Addressing

• IPv4 defines a 32 bit address
• The finite number of IP addresses will eventually
  be exhausted
• More than half of all possible IPv4 addresses
  have been assigned to ISPs, corporation,
  government agencies
• An estimated 69 million address are actually in
  use

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IP Addressing

• The long-term solution to IPv4 addressing problem
  is found in IPv6
• IPv6 uses 64 bit address

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IPv4 Addressing What is an IP address

• Network ID
• Shared or common to all computers on the same
  physical segment
• Unique on the entire network
• Area Code

• Host ID
• Identifies a specific device (Host) within a
  physical segment
• Unique on the physical segment
• Phone Number

192.176.12.50
12
IP Addressing Classes

• IP address space is divided into 3 classes A, B,
  C
• Each class fixes the boundary between the network
  ID and the host ID a different point within the
  32 bit addresses.
• Address classes specify network sizes (number of
  hosts)

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IP Addressing Classes

• Class A
• Network . Host . Host . Host
• Class B
• Network . Network . Host . Host
• Class C
• Network . Network . Network . Host

14
IP Addressing Class A

• Highest order bit set to 0 and 7 bit number
  followed by 23 bit host number
• In Binary
• Any address that starts with a 0 in the first
  bit!
• First Class A Network Address
• 00000001.00000000.0000000.00000000 (Binary)
• 1.0.0.0 (Decimal)
• Last Class A Network Address
• 01111111.00000000.00000000.00000000 (Binary)
• 127.0.0.0 (Decimal) (Loopback Address)

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Class A Networks Host ID Addresses

• 44.0.0.0 (An Assigned Class A Address)
• All devices would share the 44 network ID.
• The administrator would number the IP devices
• 44.0.0.1 44.0.0.255 (255 Addresses)
• 44.0.1.0 44.0.1.255 (256 Addresses)
• 44.0.255.0 --44.0.255.255 (256 Addresses)

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Class A Networks Host ID Addresses

• 44.0.255.0 --44.0.255.255 (256 Addresses)
• (A Total of 65,535 Addresses)
• 44.1.0.0 -- 44.1.255.255 (65,536 Addresses)
• 44.2.0.0 -- 44.2.255.255 (65,536 Addresses)
• 44.255.0.0 -- 44.255.255.254 (65,535 Addresses)
• ( Total Addresses 16.7 Million)

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IP Addressing Class B

• Any address that starts with a 10 in the first
  two bits of the first octet!
• First Class B Network Address
• 10000000.00000000.0000000.00000000 (Binary)
• 128.0.0.0 (Decimal)
• Last Class B Network Address
• 10111111.11111111.00000000.00000000 (Binary)
• 191.255.0.0 (Decimal)

18
IP Addressing Class B

• 1st and 2nd Octets are the Network IDs
• 128.0.0.0 ---128.1.0.0
• 191.255.0.0
• 3rd, 4th Octets are the Host IDs
• An Assigned Class B Network Addresses
• 131.11.0.0
• 3rd, 4th Octets are the Host IDs

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IP Addressing Class C

• In Binary
• Any address that starts with a 110 in the first
  three bits of the first octet!
• First Class C Network Address
• 11000000.00000000.0000000.00000000 (Binary)
• 192.0.0.0 (Decimal)
• Last Class C Network Address
• 11011111.11111111.11111111.00000000 (Binary)
• 223.255.255.0 (Decimal)

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IP Addressing

• Class A (1st Octet 1-127)
• Network.Host.Host.Host
• Class B (1st Octet 128-191)
• Network.Network.Host.Host
• Class C (1st Octet 192-223)
• Network.Network.Network.Host

21
Address Class Summary
22
Subnets
223.1.1.1

• IP address
• subnet part (high order bits)
• host part (low order bits)
• Whats a subnet ?
• device interfaces with same subnet part of IP
  address
• can physically reach each other without
  intervening router

223.1.2.1
223.1.1.2
223.1.2.9
223.1.1.4
223.1.2.2
223.1.1.3
223.1.3.27
subnet
223.1.3.2
223.1.3.1
network consisting of 3 subnets
23
Subnets

• Recipe
• To determine the subnets, detach each interface
  from its host or router, creating islands of
  isolated networks. Each isolated network is
  called a subnet.

Subnet mask /24
24
What is Subnet Mask

• An Address the accompanies an IP address that
  indicates which portion of the IP address is the
  Network ID and which portion of the IP address is
  the Host ID.
• 131.1.102.7 (IP Address)
• 255.255.0.0 (Subnet Mask)
• The IP Address and Subnet Mask are interrelated
  and each only has meaning in the context of the
  other!
• IP Address and SNM are the minimum IP addressing
  requirements.

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What Makes up a Subnet Mask (SNM)?

• In Binary
• 1s represent what portion of the IP address is
  the Network ID
• 0s represent what portion of the IP address is
  the Host ID
• For Example
• 131.1.102.7 (Class B Address)
• Net . Net . Host . Host
• 11111111 . 11111111 . 00000000. 00000000 (SNM in
  Binary)
• 255.255.0.0 (SNM in Decimal)