For each of the preceding ping commands, provide an explanation of why the command succeeds or fails.

Question

What will be an ideal response?

Answer 1

The ping command PC3% ping –c 3 10.0.1.3
is successful as both the hosts are in the same private network and thus share the same network ID.
```
No. Time Source Destination Protocol Info
13 39.376736 10.0.1.2 10.0.1.3 ICMP Echo (ping)
request
14 39.377580 10.0.1.3 10.0.1.2 ICMP Echo (ping)
reply
16 40.386200 10.0.1.2 10.0.1.3 ICMP Echo (ping)
request
17 40.387017 10.0.1.3 10.0.1.2 ICMP Echo (ping)
reply
18 41.386200 10.0.1.2 10.0.1.3 ICMP Echo (ping)
request
19 41.387028 10.0.1.3 10.0.1.2 ICMP Echo (ping)
```
The ping command
PC3% ping –c 3 128.143.136.1
is successful as the NAT router makes a static NAT entry of the host 10.0.1.2 in its tables with the
inside global address of 200.0.0.2. Thus all the packets originating from the host 10.0.1.2 (in our
case ICMP Echo Request packet) have their source IP addresses changed to 200.0.0.2 as they
leave the private network. Similarly all the incoming packets (ICMP Echo Response)
corresponding to the ping request have their destination IP address changed back to 10.0.1.2 by
the NAT router so that the packets can be delivered to the appropriate host.
```
155 512.103510 128.195.7.1 10.0.1.2 ICMP Echo (ping)
request
158 512.104194 10.0.1.2 128.195.7.1 ICMP Echo (ping)
reply
159 513.114800 128.195.7.1 10.0.1.2 ICMP Echo (ping)
request
160 513.114832 10.0.1.2 128.195.7.1 ICMP Echo (ping)
reply
161 514.114802 128.195.7.1 10.0.1.2 ICMP Echo (ping)
request
162 514.114832 10.0.1.2 128.195.7.1 ICMP Echo (ping)
reply
```
Ethereal dump on PC4
```
No. Time Source Destination Protocol Info
16 103.167682 200.0.0.2 128.143.136.1 ICMP Echo (ping)
17 103.167768 128.143.136.1 200.0.0.2 ICMP Echo (ping)
reply
18 104.176842 200.0.0.2 128.143.136.1 ICMP Echo (ping)
request
19 104.176876 128.143.136.1 200.0.0.2 ICMP Echo (ping)
reply
```
The ping command
Router3# ping 10.0.1.2
is successful as the destination lies in the same local network. Please refer to PC4 dump:
```
103 368.449304 10.0.1.3 10.0.1.2 ICMP Echo (ping)
request
104 368.449367 10.0.1.2 10.0.1.3 ICMP Echo (ping)
reply
105 368.450458 10.0.1.3 10.0.1.2 ICMP Echo (ping)
request
106 368.450466 10.0.1.2 10.0.1.3 ICMP Echo (ping)
reply
107 368.451495 10.0.1.3 10.0.1.2 ICMP Echo (ping)
request
108 368.451504 10.0.1.2 10.0.1.3 ICMP Echo (ping)
reply
109 368.452594 10.0.1.3 10.0.1.2 ICMP Echo (ping)
request
110 368.452602 10.0.1.2 10.0.1.3 ICMP Echo (ping)
reply
111 368.453631 10.0.1.3 10.0.1.2 ICMP Echo (ping)
request
112 368.453639 10.0.1.2 10.0.1.3 ICMP Echo (ping)
reply
```
The command
Router3# ping 128.143.136.1
that was not succeeding before will succeed now as the NAT router has an entry for router 3 thus mapping its IP address 10.0.1.3 to the inside global address 200.0.0.3.
Before:
```
122 389.871352 10.0.1.3 128.143.136.1 ICMP Echo (ping)
request
124 391.871230 10.0.1.3 128.143.136.1 ICMP Echo (ping)
request
125 393.871215 10.0.1.3 128.143.136.1 ICMP Echo (ping)
request
126 395.871137 10.0.1.3 128.143.136.1 ICMP Echo (ping)
request
```
The command
PC4% ping –c 3 10.0.1.2
still won’t succeed as there is no routing entry for the network 10.0.1.2 in the routing table of PC4.
And as such the host 10.0.1.2 is a host inside the private network not visible to the machines in the
public network.
The last ping command:
PC4% ping -c 3 200.0.0.2
is successful as there is an entry for the network 200.0.0.2/24 in PC4’s routing table. It thus forwards these packets to Router 2. Router 2 is a NAT router which has a NAT entry that maps the global address 200.0.0.2 to the local host 10.0.1.2 The packet therefore reaches PC3 which replies
to the Echo request. This packet when leaves the private network has its source IP address changed back to 200.0.0.2 and this is how PC4 is able to receive the reply to its Echo requests.
```
70 483.840207 128.195.7.1 200.0.0.2 ICMP Echo (ping)
request
71 483.842196 200.0.0.2 128.195.7.1 ICMP Echo (ping)
reply
72 484.851488 128.195.7.1 200.0.0.2 ICMP Echo (ping)
request
73 484.851959 200.0.0.2 128.195.7.1 ICMP Echo (ping)
reply
74 485.851489 128.195.7.1 200.0.0.2 ICMP Echo (ping)
request
75 485.851953 200.0.0.2 128.195.7.1 ICMP Echo (ping)
reply
```