Ubuntu cannot ping its neighbor

"sudo dhclient eth0" will provide an address if a dhcp server is running.

you can define this in /etc/network/interfaces on older versions and somewhere in /etc/netplan or using the ethtool command on newer ones. you will find good docs if you google it.

Correct deduction there is no IP address.

Check the /etc/network directory for    the interfaces file

it should have something along:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet dhcp
    inet6 dhcpv6

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To obtain an DHCP address.

Thank you very much for your fast reply.
Router1(R1) cannot see any info if using show ip arp. Then I add dhcp pool on R1, the NetworkAutomation-1 still cannot get ip address. So I think adding static ip address on eth0 might be a better way for the two devices can ping each other.

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

auto eth0
iface eth0 inet
    address 192.168.1.125
    netmask 255.255.255.0
    gateway 192.168.1.1
    inet6 dhcpv6

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Better choose a different network from 192.168.1.0/24 try one of the other 256 network available. (random from the range).

did you run dhclient or restart the network after adding the pool ?

static and dhcp should both work. just a matter of choice and we have no idea regarding your setup.

whatever config you use, check what happened with ifconfig

side notes :
- ubuntu is switching to netplan so /etc/network/interfaces may not work as expected
- ifup/ifdown commands have been broken in ubuntu since ubuntu 14 so do not rely on them too much

Why the Networkautomation-1 cannot ping itself 192.168.0.2? Is this normal?  it cannot ping its neighbor (192.168.0.1) too. The ip address (192.168.0.2) looks like to be static ip address. Is this default ip address? 

did you run dhclient or restart the network after adding the pool ?

yes, I tried it. but i could not see any difference.

unless you restart the network, making changes to the file does nothing.

check the current ip with ifconfig or "ip a"

you most definitely should be able to ping yourself if the ip is actually set.

Please see below. this eth0 does not show any ipv4 address. What command I can use to add a static ipv4 address to eth0?

root@NetworkAutomation-2:~# ifconfig
eth0: flags=4163<UP,BROADCAST,RUNNING,MULTICAST>  mtu 1500
        inet6 fe80::884f:b1ff:fe24:5d49  prefixlen 64  scopeid 0x20<link>
        ether 8a:4f:b1:24:5d:49  txqueuelen 1000  (Ethernet)
        RX packets 189  bytes 19046 (19.0 KB)
        RX errors 0  dropped 154  overruns 0  frame 0
        TX packets 14  bytes 1076 (1.0 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

lo: flags=73<UP,LOOPBACK,RUNNING>  mtu 65536
        inet 127.0.0.1  netmask 255.0.0.0
        inet6 ::1  prefixlen 128  scopeid 0x10<host>
        loop  txqueuelen 1000  (Local Loopback)
        RX packets 48  bytes 3264 (3.2 KB)
        RX errors 0  dropped 0  overruns 0  frame 0
        TX packets 48  bytes 3264 (3.2 KB)
        TX errors 0  dropped 0 overruns 0  carrier 0  collisions 0

if there is a # mark on a line anything after it is Comment and not actualy used.
Remove the # marks  on lines whter the are not needed. (Compare to my previous comment).

Hi noci, Can you share how to enter that status or remove the # mark? Thank you.

lol, missed that !
use a text editor

then resart the network or ifdown and ifup and check the results with ifconfig

i did it, but nothing happened 

Edit the file into:

# This file describes the network interfaces available on your system
# and how to activate them. For more information, see interfaces(5).

# The loopback network interface
auto lo
iface lo inet loopback

#the Ethernet adapter.
auto eth0
iface eth0 inet
    address 192.168.0.2
    netmask 255.255.255.0
    gateway 192.168.0.1
#    inet6 dhcpv6

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Note that will NOT talk to your router that you had setup with 192.168.1.1 in an earlier image.

two possible issues

your ubuntu version might be using netplan so changes to this file might be ignored

you messed up with the command or hit a bug. many recent ubuntu versions are plagued with bugs in network restart or ifup and down

you can setup an ip manually till you figure it out ( we can help if you post commands and output and the ubuntu version. use lsb_release -a ) using "ifconfig eth0 192.whatever"

root@NetworkAutomation-2:~# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 18.04.4 LTS
Release:        18.04
Codename:       bionic

I noticed even if I removed the # and then save/refresh, i still cannot see ipv4 address on eth0 using ifconfig. As the interface eth0 does not have ip address, it should not ping to itself. So what i might  need to do is to add the ip address. but it looks like the ip address cannot be added. Why cannot?

To do manual for testing....

ifconfig eth0 192.168.0.2 netmask 255.255.255.0
route add default gw 192.168.0.1
ping 192.168.0.1
traceroute 1.1.1.1

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Please show your CURRENT interfaces file (text format, not an image).

Below is configuration change. Is it possible for us to see the added static ip address when using ifconfig? now I cannot see it after making the change

Great! Thank you noci

Addition question: right click on Networkautomation-1 ----> Edit config. When i change it, why the change cannot work. for example, i change its ip address on eth0, but i cannot see the change when issuing command ifconfig

After setting the config you need to either stop/start the interface on command line:

ifdown eth0
ifup eth0

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Alternatively:
reboot the system.

When the commands have been given manual then after restart of the system the settings are LOST.