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P-2302R Series User’s Guide

Chapter 7 Introduction to VoIP 96

7.3.2.1 Full Cone NAT

In full cone NAT, the NAT router maps all outgoing packets from an internal IP address and

port to a single IP address and port on the external network. The NAT router also maps

packets coming to that external IP address and port to the internal IP address and port.

In the following example, the NAT router maps the source address of all packets sent from the

Prestige’s internal IP address 1 and port A to IP address 2 and port B on the external network.

The NAT router also performs NAT on all incoming packets sent to IP address 2 and port B

and sends them to IP address 1, port A.

Figure 32 Full Cone NAT Example

7.3.2.2 Restricted Cone NAT

As in full cone NAT, a restricted cone NAT router maps all outgoing packets from an internal

IP address and port to a single IP address and port on the external network. In the following

example, the NAT router maps the source address of all packets sent from internal IP address

1 and port A to IP address 2 and port B on the external network.

The difference from full cone NAT is in how the restricted cone NAT router handles packets

coming in from the external network. A host on the external network (IP address 3 or IP

address 4 for example) can only send packets to the internal host if the internal host has

already sent a packet to the external host’s IP address.

A Prestige with IP address 1 and port A sends packets to IP address 3 and IP address 4. The

NAT router changes the Prestige’s IP address to 2 and port to B.

Both 4, D and 4, E can send packets to 2, B since 1, A has already sent packets to 4. The NAT

router will perform NAT on the packets from 4, D and 4, E and send them to the Prestige at IP

address 1, port A. Packets have not been sent from 1, A to 3 or 5, so 3 and 5 cannot send

packets to 1, A.

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