ZyXEL P-2002 User Guide - Page 51

Restricted Cone NAT

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Figure 19 Full Cone NAT Example P-2002 Series User's Guide 5.3.2.2 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. Figure 20 Restricted Cone NAT Example Chapter 5 Introduction to VoIP 51

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P-2002 Series User’s Guide
Chapter 5 Introduction to VoIP
51
Figure 19
Full Cone NAT Example
5.3.2.2
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
.
Figure 20
Restricted Cone NAT Example