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


 
Configuring IP Addressing
Configuring Network Address Translation
IPC-40
Cisco IOS IP Configuration Guide
Figure 5 NAT Overloading Inside Global Addresses
The router performs the following process in overloading inside global addresses, as shown in Figure 5.
Both host B and host C believe they are communicating with a single host at address 2.2.2.2. They are
actually communicating with different hosts; the port number is the differentiator. In fact, many inside
hosts could share the inside global IP address by using many port numbers.
1. The user at host 1.1.1.1 opens a connection to host B.
2. The first packet that the router receives from host 1.1.1.1 causes the router to check its NAT table:
If no translation entry exists, the router determines that address 1.1.1.1 must be translated, and
sets up a translation of inside local address 1.1.1.1 to a legal global address.
If overloading is enabled, and another translation is active, the router reuses the global address
from that translation and saves enough information to be able to translate back. This type of
entry is called an extended entry.
3. The router replaces the inside local source address 1.1.1.1 with the selected global address and
forwards the packet.
4. Host B receives the packet and responds to host 1.1.1.1 by using the inside global IP address 2.2.2.2.
5. When the router receives the packet with the inside global IP address, it performs a NAT table
lookup, using the protocol, inside global address and port, and outside address and port as a key;
translates the address to inside local address 1.1.1.1; and forwards the packet to host 1.1.1.1.
Host 1.1.1.1 receives the packet and continues the conversation. The router performs Steps 2 through 5
for each packet.
To configure overloading of inside global addresses, use the following commands in global
configuration mode:
1.1.1.2:1723
1.1.1.1:1024
2.2.2.2:1723
2.2.2.2:1024
Inside Local IP
address:port
TCP
TCP
Protocol Inside Global IP
address:port
6.5.4.7:23
9.6.7.3:23
Outside Global
IP address:port
1.1.1.2
Inside
NAT table
3
SA
2.2.2.2
5
DA
1.1.1.1
1.1.1.1
1
4
DA
2.2.2.2
4
DA
2.2.2.2
2
Host B
9.6.7.3
Host C
6.5.4.7
S4791
Internet
SA
1.1.1.1
Command Purpose
Step 1
Router(config)# ip nat pool name start-ip end-ip
{netmask netmask | prefix-length prefix-length}
Defines a pool of global addresses to be allocated as
needed.
Step 2
Router(config)# access-list access-list-number
permit source [source-wildcard]
Defines a standard access list.