NAT Router Address Translation Process | Generated by AI
Question: In the diagram (Figure 32), Host A on a private network sends datagrams to Host B on the Internet. Please analyze why a NAT router is used, and identify the IP addresses at positions ①, ②, ③, and ④.
Answer:
Why a NAT Router is Used
Reason 1: Private IP Addresses Cannot Route on the Public Internet
Host A uses the private IP address 192.168.0.3, which belongs to the 192.168.0.0/24 private address range. Private addresses (defined by RFC 1918) are:
10.0.0.0/8172.16.0.0/12192.168.0.0/16
These addresses are not routable on the public Internet — routers on the Internet will drop packets with private source addresses.
Reason 2: Address Translation is Necessary for External Communication
For Host A to communicate with Host B (213.18.2.4) on the Internet, its private address must be translated into a globally routable public IP address. This process is called Network Address Translation (NAT).
Reason 3: NAT Router Performs the Translation
The NAT router sits at the boundary between the private network and the Internet. It:
- Replaces the private source address with its own public (global) IP address (172.38.1.5) when forwarding packets outbound
- Reverses the translation for inbound reply packets, forwarding them back to Host A
IP Addresses at Each Position
| Position | Direction | Field | IP Address | Explanation |
|---|---|---|---|---|
| ① | Outgoing (inside private net) | Source IP | 192.168.0.3 | Host A’s private address is the source |
| ② | Return (inside private net) | Destination IP | 192.168.0.3 | Reply packet must be delivered back to Host A |
| ③ | Outgoing (toward Internet) | Source IP | 172.38.1.5 | NAT router replaces private source with its public IP |
| ④ | Return (toward Internet) | Destination IP | 172.38.1.5 | Host B sends reply to the NAT router’s public IP |
Summary of NAT Translation Process
Host A (192.168.0.3) ──► NAT Router ──► Internet ──► Host B (213.18.2.4)
Source: 192.168.0.3 Source becomes: 172.38.1.5
Dest: 213.18.2.4 Dest remains: 213.18.2.4
Host A (192.168.0.3) ◄── NAT Router ◄── Internet ◄── Host B (213.18.2.4)
Dest: 192.168.0.3 Dest becomes: 192.168.0.3
Src: 213.18.2.4 Src remains: 213.18.2.4
The NAT router maintains a translation table to track which internal host corresponds to each active connection, enabling correct delivery of return packets.