Can't connect to computer on LAN

[tjohnson_nb]

Strangest thing has happened. I have a workstation and a laptop on the same network both running 16.04 and all of a sudden I can't ping the workstation from the laptop. I can ping the router from the laptop and I can ping the workstation from the router but I can't ping the workstation from the laptop. additionally I have another laptop also running 16.04 and it no problem.Not sure sure how to troubleshoot this

[jlittle]

What's the router?

There have been models that would do this for no reason; I had a Linksys that did it. Anyway, a router perspective might offer clues.

[oshunluvr]

When you say "all of a sudden" you mean you've made no changes to settings on either system? Both PCs have to have to be on the same subnet and in the same IP range to communicate. Assuming that's the case, try some of these commands (you may have to install one or more of these tools):

arp -e
nmap -sn 192.168.1.0/24
sudo arp-scan -l

For the nmap command, use your own inet range.

If any of these show the missing PC, it's on the network.

If they're on the same network and can't see each other, the most likely culprit is a firewall or iptable misconfiguration.

[vinnywright]

When you say "all of a sudden" you mean you've made no changes to settings on either system? Both PCs have to have to be on the same subnet and in the same IP range to communicate. Assuming that's the case, try some of these commands (you may have to install one or more of these tools):

arp -e
nmap -sn 192.168.1.0/24
sudo arp-scan -l

For the nmap command, use your own inet range.

If any of these show the missing PC, it's on the network.

If they're on the same network and can't see each other, the most likely culprit is a firewall or iptable misconfiguration.

nice I was unaware of that arp command and was looking just 2-3 days ago for something to quickly show me the IP addresses of the computers on my home LAN

I know about nmap but did not think to use it for something so simple , but works great for this as well .

however on the arp one ,,,,,,,, wile reading the man page , I saw no -e switch and in fact get the same output with or without it .

vinny@vinny-Bonobo-Extreme:~$ arp
Address                  HWtype  HWaddress           Flags Mask            Iface
192.168.0.4              ether   xx:xx:xx:xx:xx:xx   C                     wlp4s0
192.168.0.1              ether   xx:xx:xx:xx:xx:xx   C                     wlp4s0
vinny@vinny-Bonobo-Extreme:~$ arp -e
Address                  HWtype  HWaddress           Flags Mask            Iface
192.168.0.4              ether    xx:xx:xx:xx:xx:xx  C                     wlp4s0
192.168.0.1              ether    xx:xx:xx:xx:xx:xx  C                     wlp4s0

however arp is not showing the box I am running it from , just the router and the box thats always connected to the TV's side HDMI input , for media viewing or listening to

the nmap one gives them all ,even the one its ran from

vinny@vinny-Bonobo-Extreme:~$ nmap  -sn 192.168.0.1/24

Starting Nmap 7.01 ( https://nmap.org ) at 2017-04-06 22:15 EDT
Nmap scan report for 192.168.0.1
Host is up (0.019s latency).
Nmap scan report for 192.168.0.4
Host is up (0.013s latency).
Nmap scan report for 192.168.0.17
Host is up (0.000057s latency).
Nmap done: 256 IP addresses (3 hosts up) scanned in 2.54 seconds

VINNY

[oshunluvr]

Yeah, the -e was a mistake. Should have been -a for all but it looks like the default output is easier to read. The -a gives the ipaddress of the device also. It doesn't list your device because it's showing what your device is connected to. The "Iface" column on the right lists your network connection(s) - so your w1p4s0 interface isn't connected to itself - that would be silly.

[vinnywright]

- so your w1p4s0 interface isn't connected to itself - that would be silly.

,,,yup ,,gotcha

VINNY