Network Address Translation Network Address Translation, NAT, is the translation
ID: 3569738 • Letter: N
Question
Network Address Translation
Network Address Translation, NAT, is the translation of an Internet Protocol address
(IP address) used within one network to a different IP address known within another
network.
a. Why do we need NAT? Why is it a service that is present on many routers
today?
b. NAT is considered to be a temporary solution. What technology when deployed
will do away with the need of NAT?
Recall, a NAT capable router essentially translates private address within a network to
public addresses that can be used publicly. In this question, we try and understand the
working behind a simple NAT capable router. A simple NAT capable router will have
mappings between the private addresses within the network (here, the privateaddresses all fall within the 10/8 network) to the public address(es) that it uses. Let us
assume that the router has a single public address 138.76.29.7 which it uses for all
communication with hosts that are not part of the private network. The router
multiplexes its public IP address(es) as needed and keeps track of the multiplexing in
a NAT translation table.
Assume that the router multiplexes the public address using ports starting from 8000
and then incrementing by one. For example, if a host in the private network with
address 10.0.0.5:5000 sends a message to 132.239.8.45:80 then the entry in
the NAT table would be filled in as below. The next time the router will use 8001 as
the port to establish a new connection and so on.
c. What would be the entries in the NAT Translation Table at the end of the
following events
i. 10.0.0.6:5000 sends a message to 74.125.239.33:80
ii. 10.0.0.10:6000 sends a message to 204.79.197.200:80
iii. 10.0.1.101:6001 sends a message to 206.190.36.45:80
iv. 10.0.0.10:6000 sends another message to 204.79.197.200:80
v. 10.0.1.101:6001 sends a message to 74.125.239.33:80
vi. 10.0.0.7:7000 sends a message to 63.245.215.20:80
vii. 204.79.197.200:80 sends a message to 10.0.0.10:6000
viii. 206.190.36.45:80 sends a message to 74.125.239.33:80
Note: The NAT Table should like the one above with the entries generated from
the events in (c.) filled in
d. For simplicity, let us assume that message format is MSG Receiver>. In that case, if a host in the private network with address
10.0.0.5:5000 sends a message to 132.239.8.45:80 then the message
recevied at the router and leaving at the router would look as follows
Message Received from Host: MSG<10.0.0.5:5000, 132.239.8.45:80>
Message Sent from Router: MSG<138.76.29.7:8000,132.239.8.45:80>
Note: We will need to use the entries from the table we filled in (c.) to do this.List out the message received from the host at the router and the message sent
from the router (like shown above) for the following messages:
1. 10.0.0.6:5000sends a message to 74.125.239.33:80
2. 10.0.0.10:6000sends a message to 204.79.197.200:80
e. If the router gets a message
MSG<74.125.239.33:80,138.76.29.7:8001>, what would the message
look like leaving the router?
f. Finally, if the router gets a message MSG<10.0.0.10:6000,
10.0.1.101:6001>, what what would the message look like leaving the
router?
Explanation / Answer
NAT is the handshaker. It shakes hands with the WAN so it can decode it for LAN.
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