Cisco Systems 78-11741-02 Wireless Office Headset User Manual


 
Configuring IP Addressing
Configuring Network Address Translation
IPC-42
Cisco IOS IP Configuration Guide
Figure 6 NAT Translating Overlapping Addresses
The router performs the following process when translating overlapping addresses:
1. The user at host 1.1.1.1 opens a connection to host C by name, requesting a name-to-address lookup
from a DNS server.
2. The router intercepts the DNS reply and translates the returned address if there is an overlap (that
is, the resulting legal address resides illegally in the inside network). To translate the return address,
the router creates a simple translation entry mapping the overlapping address 1.1.1.3 to an address
from a separately configured, outside local address pool.
The router examines every DNS reply from everywhere, ensuring that the IP address is not in the
stub network. If it is, the router translates the address.
3. Host 1.1.1.1 opens a connection to 3.3.3.3.
4. The router sets up translations mapping inside local and global addresses to each other, and outside
global and local addresses to each other.
5. The router replaces the SA with the inside global address and replaces the DA with the outside
global address.
6. Host C receives the packet and continues the conversation.
7. The router does a lookup, replaces the DA with the inside local address, and replaces the SA with
the outside local address.
8. Host 1.1.1.1 receives the packet and the conversation continues, using this translation process.
1.1.1.1
DNS request for host C address
SA=1.1.1.1 DA=x.x.x.x
DNS server
x.x.x.x
2.2.2.2 1.1.1.3
Inside Global
IP Address
NAT table
Outside Global
IP Address
3.3.3.3
Outside Local
IP Address
Host C
1.1.1.3
S4792
Internet
1.1.1.1
Inside Local
IP Address
DNS request for host C address
SA=2.2.2.2 DA=x.x.x.x
DNS response from x.x.x.x
SA=x.x.x.x DA=1.1.1.1
C=3.3.3.3
1.1.1.1 message to host C
SA=1.1.1.1 DA=3.3.3.3
DNS response from x.x.x.x
SA=x.x.x.x DA=2.2.2.2
C=1.1.1.3
1.1.1.1 message to host C
SA=2.2.2.2 DA=1.1.1.3