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Port Address Translation: saving IP addresses

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• Home networking
  • When you want to access the Internet using multiple computers on a site, you must assign each computer a unique IP address

    Example:

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  • Problem:

    As more and more households are connected to the Internet, and each computer must be given a unique IP address, the IPv4 addresses is be depleted very quickly --- unless we take measure to re-use IPv4 addresses

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• Private IP addresses
  • The Internet Engineering Task Force (IETF) has designated a set of IP addresses as private addresses:

  • Private IP network:

    An IP network that uses these private IP addresses is called a private IP network

  • Use of private address (non-routable address):

    Private IP addresses are commonly used for home office enterprise local area networks (LANs) Anywhere when globally routable addresses are not mandatory

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  • Example: 2 private networks

    NOTE:

    Home private networks usually use the 192.168.x.x block of addresses In the figure, 2 privaye IP networks are given. Notice that different nodes are using the same IP address !!!

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  • Private IP address spaces were originally defined in an effort to delay IPv4 address exhaustion

    They are also a feature of IPv6.

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• Naive private networking problem
  • Nodes on a private network cannot use the Internet naively

    (Naive here means without some additional "help" - "plainly")

  • The problem:

    IP routing requires that each IP host be uniquely identified by its IP address We saw that private IP addresses do not uniquely identify one IP host on the Internet I.e., private IP addresses are not globally unique

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    Example: you can have two identical TCP connections

    The IP routing mechanism cannot deliver the messages correctly to the intended receiver !!!

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• Port Address Translation
  • PAT Translation Table:

    A PAT translation table maps a (Private IP address, Port #) to (Public IP address, Port #)

  • Port Address Translation: (picture taken from Cisco)

    Note:

    The private IP address:port# (10.10.10.1:80) is mapped to the public IP address:port# (63.63.63.1:8000) The private IP address:port# (10.10.10.2:80) is mapped to the public IP address:port# (63.63.63.1:8001) The private IP address:port# (10.10.10.3:80) is mapped to the public IP address:port# (63.63.63.1:8002)

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  • How PAT works:

    Note:

    The router intercepts a packet containing a source private IP address : Port# It replaces the (source) private IP address : Port# with the corresponding public IP address : Port# After replacing the private address, it will forward the IP packet on the Internet

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  • When the receiver replies back, the router will replace the public IP address : Port# in the destination with the corresponding private IP address : Port#:

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  • Net result:

    PAT allows computers in a private network to access the Internet In fact, multiple computers on the private network can access the Internet using one single IP address (namely, the public IP address of the router (24.0.0.4)