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TCP/IP Tutorial and Technical Overview - IBM Redbooks

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As shown in Figure 3-14, Network Address Port Translation is able to translate<br />

many network addresses <strong>and</strong> their transport identifiers into a single network<br />

address with many transport identifiers, or more specifically, ports.<br />

a.b.65.3 /30<br />

Figure 3-14 Network Address Port Translation<br />

NAPT maps private addresses to a single globally unique address. Therefore,<br />

the binding is from the private address <strong>and</strong> private port to the assigned address<br />

<strong>and</strong> assigned port. NAPT permits multiple nodes in a local network to<br />

simultaneously access remote networks using the single <strong>IP</strong> address assigned to<br />

their router.<br />

In NAPT, modifications to the <strong>IP</strong> header are similar to that of Basic NAT.<br />

However for <strong>TCP</strong>/UDP sessions, modifications must be extended to include<br />

translation of the source port for outbound packets <strong>and</strong> destination port for<br />

inbound packets in the <strong>TCP</strong>/UDP header. In addition to <strong>TCP</strong>/UDP sessions,<br />

ICMP messages, with the exception of the REDIRECT message type, can also<br />

be monitored by the NAPT service running on the router. ICMP query type<br />

packets are translated similar to that of <strong>TCP</strong>/UDP packets in that the identifier<br />

field in ICMP message header will be uniquely mapped to a query identifier of the<br />

registered <strong>IP</strong> address.<br />

NAT limitations<br />

The NAT limitations are mentioned in RFC 3022 <strong>and</strong> RFC2663. We discuss<br />

some of the limitations here.<br />

NAT works fine for <strong>IP</strong> addresses in the <strong>IP</strong> header. Some application protocols<br />

exchange <strong>IP</strong> address information in the application data part of an <strong>IP</strong> packet, <strong>and</strong><br />

NAT will generally not be able to h<strong>and</strong>le translation of <strong>IP</strong> addresses in the<br />

application protocol. Currently, most of the implementations h<strong>and</strong>le the FTP<br />

protocol. It should be noted that implementation of NAT for specific applications<br />

that have <strong>IP</strong> information in the application data is more sophisticated than the<br />

st<strong>and</strong>ard NAT implementations.<br />

94 <strong>TCP</strong>/<strong>IP</strong> <strong>Tutorial</strong> <strong>and</strong> <strong>Technical</strong> <strong>Overview</strong><br />

Transition Table<br />

10.10.10.11:80 = a.b.65.1:8000<br />

10.10.10.12:80 = a.b.65.1:8001<br />

External Internal<br />

a.b.65.0 /30 10.10.10.0 /24<br />

NAPT<br />

10.10.10.11 /24<br />

10.10.10.12 /24

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