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Private Network Addresses

Private Network Addresses. IP addresses in a private network can be assigned arbitrarily. Not registered and not guaranteed to be globally unique Generally, private networks use addresses from the following experimental address ranges ( non-routable addresses ): 10.0.0.0 – 10.255.255.255

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Private Network Addresses

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  1. Private Network Addresses • IP addresses in a private network can be assigned arbitrarily. • Not registered and not guaranteed to be globally unique • Generally, private networks use addresses from the followingexperimental address ranges (non-routable addresses): • 10.0.0.0 – 10.255.255.255 • 172.16.0.0 – 172.31.255.255 • 192.168.0.0 – 192.168.255.255

  2. NAT • NAT is a router function where IP addresses (and possiblyport numbers) of IP datagrams are replaced at the boundaryof a private network • NAT is a method that enables hosts on private networks tocommunicate with hosts on the Internet • NAT is run on routers that connect private networks to thepublic Internet, to replace the IP address-port pair of an IPpacket with another IP address-port pair.

  3. Mapping Out • 10.x.y.z source address in private network is replaced by a true company IP address • TCP source port field is replaced by an index into the NAT box’s 65536 entry address translation table. • Translation table contains original IP address and source port. • IP and TCP checksums are recomputed

  4. Mapping Inwards • Incoming packet at NAT box, from ISP • Source port in TCP header extracted, used as index into NAT box’s mapping table. • Internal IP address and original source port inserted into packet. • Checksums recomputed, and packet sent to router.

  5. Placement of NAT Box

  6. Operation of NAT and Address Translation

  7. Pooling IP Addresses • Scenario: Corporate network has has many hosts but only asmall number of public IP addresses • NAT solution:– Corporate network is managed with a private addressspace • NAT device, located at the boundary between thecorporate network and the public Internet, manages a poolof public IP addresses • When a host from the corporate network sends an IPdatagram to a host in the public Internet, the NAT devicepicks a public IP address from the address pool, and bindsthis address to the private address of the host.

  8. MigrationBetween ISPs • Scenario: In CIDR, the IP addresses in a corporate network are obtainedfrom the service provider. • Changing the service provider requireschanging all IP addresses in the network. • NAT solution:– Assign private addresses to the hosts of the corporate network • NAT device has static address translation entries which bind theprivate address of a host to the public address. • Migration to a new network service provider merely requires an updateof the NAT device. The migration is not noticeable to the hosts on thenetwork. • Note:The difference to the use of NAT with IP address pooling is that themapping of public and private IP addresses is static.

  9. An abomination ? • IP architectural model violated. IP address now does not uniquely identify a device. • Internet now connection oriented with mapping tables. • Layering rule violated as layer independance destroyed. • NAT boxes require TCP source ports and so is reliant on TCP • Some apps, like FTP insert IP addresses into payload, NAT cannot see this and so apps fail.

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