chap 8

• Outline topics

  • IPv4 Addressing Space and its limitation
  • Public and Private IP Addresses
  • What is Network Address Translation (NAT)?
  • Types of NAT
  • Static NAT
  • Dynamic NAT
  • Overloading (PAT)
  • Broadband Router’s NAT Configuration
  • How does PAT work?
• IPv4

  Uses 32 bits for addressing
• 32 bits gives 2^32 = 4.3 billion different IP addresses
• Reserved IP addresses

  • Local loopback IP addresses
  • Class D IP addresses
  • Class E IP addresses
• Local loopback IP address

  • 127.x.x.x
• Class D IP address range

  • 224.0.0.0 to 239.255.255.255
• Class E IP address range

  • 240.x.x.x – 254.x.x.x
• Available IPv4 addresses (Classes A, B and C) ≈ 3.7 billion
• 3.7 billion usable IP addresses are insufficient
• World population is 7 billion
• Each person may have multiple communication devices
• Temporary solution: Sharing of IP Addresses
• Long term solution is IPv6
• IP addresses must be unique for each device on the internet
• DHCP

  Only assigns IP address to a device when it is connected
• Public IP addresses

  IP addresses that routers in the internet will route
• Private IP addresses

  Can only be used in private networks
• Routers in the internet will not forward packets destined for private IP addresses
• Private IP address ranges

  • 10.x.x.x
  • 172.16.0.0 to 172.31.255.255
  • 192.168.x.x
• Private IP addresses are to be used in private networks only
• Example of a private IP address at home is 192.168.xxx.xxx
• Public IP addresses

  Assigned to internet users by ISPs
• Public IP addresses are IPv4 addresses that are not reserved for specific uses
• Network Address Translation (NAT)

  Translates private IP address to public IP address and vice versa
• NAT modifies the IP packet header
• NAT modifies these fields in the IP packet header

  • Source IP address
  • Destination IP address
• For outgoing IP packets, the router modifies the Source IP Address field

  
• For incoming IP packets, the router modifies the Destination IP Address field

  
• Replacement of Src/Dest IP Address is based on records in the Address Translation Table
• NAT is necessary because devices in different private networks may use identical private IP addresses
• Identical private IP addresses can cause IP address conflict
• Types of NAT

  • Static NAT
  • Dynamic NAT
  • Overloading (PAT)
  • Overlapping
• Static NAT

  Mapping of private-public IP addresses is fixed, one-to-one
• A private IP address is always translated to the same public IP address in Static NAT
• Dynamic NAT

  Mapping of private-public IP addresses is dynamic, depending on available public IP address
• A private IP address is translated to the first available public IP address in Dynamic NAT
• Overloading (PAT)

  All private IP addresses are translated to the same public IP address
• Overloading is used in broadband routers at home
• ISPs issue a single, temporary public IP address to your modem
• All devices in a home network use private IP addresses issued by the broadband router