TCP/IP Tutorial and Technical Overview - IBM Redbooks

As shown in Figure 3-11, Basic NAT takes the IP address of an outgoing packet
and dynamically translates it to an officially assigned global address. For
incoming packets, it translates the assigned address to an internal address.
NAT Configuration
RESERVE a.b.2.0 255.255.255.0
TRANSLATE 10.0.0.0 255.0.0.0
Non-Secure a.b.1.0/24
a.b.1.1
Figure 3-11 Basic Network Address Translation (NAT)
From the point of two hosts that exchange IP packets with each other, one in the
internal network and one in the external network, the NAT itself is transparent
(see Figure 3-12).
Non-Secure a.b.1.0/24
a.b.1.1
TCP/UDP
IP/ICMP
NAT
Filtering
src=a.b.1.1 dest=a.b.2.1 src=a.b.1.1 dest=10.0.1.1
Looks like a
normal router
src=a.b.1.1 dest=a.b.2.1
Figure 3-12 NAT seen from the external network
Filtering Rules
Based on non-translated
IP addresses (10.x.x.x)
10.0.0.0/8
Basic NAT translation mechanism
For each outgoing IP packet, the source address is checked by the NAT
configuration rules. If a rule matches the source address, the address is
translated to a global address from the address pool. The predefined address
pool contains the addresses that NAT can use for translation. For each incoming
Secure
10.0.1.1
a.b.2.0/24
Secure
a.b.2.1
Chapter 3. Internetworking protocols 91