Suppose that the host with IP address 10.0.1.23 sends an IP datagram destined to

Question

Suppose that the host with IP address 10.0.1.23 sends an IP datagram destined to host 128.119.166.186. The host’s source port is 3401, NAT router can generate a new source port number 5480, and the destination port is 80.

1. Consider the datagram at step 1, after it has been sent by the host but before it has reached the NATted router.

a. What are the source and destination IP addresses for this datagram?

b. What are the source and destination port numbers for the TCP segment in this IP datagram?

2. Now consider the datagram at step 2, after it has been transmitted by the NATted router.

a. What are the source and destination IP addresses for this datagram?

b. What are the source and destination port numbers for the TCP segment in this IP datagram?

c. Identify the differences in datagram's IP addresses and port numbers between step 1 and step 2.

d. Specify the entry that has been made in the router's NAT table. 3. Now consider the datagram at step

3, just before it is received by the NATted router.

a. What are the source and destination IP addresses for this datagram?

b. What are the source and destination port numbers for the TCP segment in this IP datagram?

4. Last, consider the datagram at step 4, after it has been transmitted by the NATted router but before it has been received by the host.

a. What are the source and destination IP address for this datagram?

b. What are the source and destination port numbers for the TCP segment in this IP datagram?

c. Identify the differences in datagram's IP addresses and port numbers between step 3 and step 4.

EQ3. 11 pts Consider the scenario below in which three hosts, with private IP addresses 10.0.1.11, 10.0.1.17, 10.0.1.23 are in a local network behind a NATted router that sits between these three hosts and the larger Internet. IP datagrams being sent from, or destined to, these three hosts must pass through this NAT router. The router's interface on the LAN side has IP address 10.0.1.28, while the router's address on the Internet side has IP address 135.122.192.214. (Before doing this problem, you might want to reread the section on the NAT protocol in section 4.3.4 in the text.) NAT translation table WAN side addr LAN side addr 10.0.1.11 10.0.1.28 2 10.0.1.17 9-135 122192214 To-70. S: D: 10.0.1.23

Explanation / Answer

Q1) answer: ------>>>>>>

a) source IP = 10.0.1.11    b) source port = 3401

   destination IP = 128.119.166.186    destination port = 80

becuase of the abstraction of the network layer it has above source - destination combination.

Q2) answer : ----------------->>>>>>>>>>>>>

a)    source Ip = 135.122.192.214 destination IP = 128.119.166.186

b) source port = assigned by nat    destination port = 80

c) here source Ip is changed to Nat ip address on the wan port because outside world does not Know the private ip address of the computer on Lan . And also the source port no. is changed and assigned by the Nat because there is many node present in the lan by doing this we make every node free to choose the port no. for their network application for communication

d) NAT Table : in wan side address it put 128.119.166.186 and in Lan side address in same row it will put 10.0.1.11

Q3) answer : -------------------------->>>>>>>>>>>>>>>>>

a) source Ip = 128.119.166.186 destination IP = 135.122.192.214

b) source Port = 80 destination Port = same as source port no. in step 2

Q4) answer : -------------------------------->>>>>>>>>>>>>>>>>>

a) source IP = 128.119.166.186 destination IP = 10.0.1.11

b) source port = 80    destination port = 3401

c)Here Ip address of the destination changed by nat by looking into their table from 135.122.192.214 to 10.0.1.11

And also the nat changed the port no. from assigned by nat to 3401 and it send that packet to the actual machine who is communicating with the server. In this way this NAT work and we have the option to choose our own private IP for our local node .

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