I had 2 main problems.

1. The major problem was that network manager was not completely disabled - this really messed up my configuration on A. Until I got this sorted there was no connectivity whatsoever. As soon as I disabled with mask (read about three levels of switching-off systemd) I got the connection. Both Arch and Manjaro pages about setting up static IP with netctl mention that you need to disable NetworkManager, but they don't quite say that some services may still bring NM up even though I disabled it at the 2nd level initially. Commands are:

   systemctl mask NetworkManager systemctl mask NetworkManager-dispatcher

2. Then I got the problem that sometimes this configuration would mysteriously stop working - i.e. plugging in the cable and setting laptop B NetworkManager profile to the static IP would give no results. Strangely the workstation A card link light is still on. Since at home with RaspberryPis this always works, I think this is some old hardware problems for the workstation. The best way to deal with this I found was to use mii-tool -w enp3s0 on the laptop. It means to watch your particular ethernet interface (enp3s0 in my case) for hardware level link. It changes instantly as you plug / disconnect your cable. ethtool seems more powerful but I did not find such convenient monitoring of the link for ethtool. So if mii-tool show the link, you look if your laptop interface is associated with the correct profile and then you can check with ping.

3. The third problem of course is that I don't do this kind of thing often - hence as an amateur I have many things to doubt, which makes the process more complicated. For that reason I will put some more bits here that I found particularly helpful:

One is lshw -class network -businfo - it gives you PCI address, device name and description. So on a machine with several interfaces you can make sure that your netctl or other CLI configuration is referring to the right card.

About IP v4 configuration. As you are setting up just 2 computers on the same subnet you do not need to set gateway. However NetworkManager GUI would not allow you to make a profile without a GW. You can just put the address of the other computer there or you can put 0.0.0.0 as GW.

Problem with the ethernet card

I do not know how, but part of the problem was with the Realtek ethernet card I had. For some unknowable reason the computer was booting each time with slightly different MAC address and most of it was fff - like ff:ff:ff:7f:ff:ff. It was frustrating as hell as I saw in logs that IP address assigning fails. I could not solve it by trying to assign a different MAC address to it at boot. This problem was solved buying another cheap used ethernet card that seemed to come up with proper MAC address every time.

Problem on laptop end

I found that sometimes my manjaro laptop could not see the network interface after cable was unplugged and then plugged in again. So the command mii-tool -w enp3s0 would complain that there was no such interface. However that was easy to solve by putting laptop to sleep and then waking it up again. mii-tool would report link and then ping would work as expected.

I had 2 main problems.

1. The major problem was that network manager was not completely disabled - this really messed up my configuration on A. Until I got this sorted there was no connectivity whatsoever. As soon as I disabled with mask (read about three levels of switching-off systemd) I got the connection. Both Arch and Manjaro pages about setting up static IP with netctl mention that you need to disable NetworkManager, but they don't quite say that some services may still bring NM up even though I disabled it at the 2nd level initially. Commands are:

   systemctl mask NetworkManager systemctl mask NetworkManager-dispatcher

2. Then I got the problem that sometimes this configuration would mysteriously stop working - i.e. plugging in the cable and setting laptop B NetworkManager profile to the static IP would give no results. Strangely the workstation A card link light is still on. Since at home with RaspberryPis this always works, I think this is some old hardware problems for the workstation. The best way to deal with this I found was to use mii-tool -w enp3s0 on the laptop. It means to watch your particular ethernet interface (enp3s0 in my case) for hardware level link. It changes instantly as you plug / disconnect your cable. ethtool seems more powerful but I did not find such convenient monitoring of the link for ethtool. So if mii-tool show the link, you look if your laptop interface is associated with the correct profile and then you can check with ping.

3. Hardware related problems - hence as an amateur I have many things to doubt, which makes the process more complicated. For that reason I will put some more bits here that I found particularly helpful:

One is lshw -class network -businfo - it gives you PCI address, device name and description. So on a machine with several interfaces you can make sure that your netctl or other CLI configuration is referring to the right card.

About IP v4 configuration. As you are setting up just 2 computers on the same subnet you do not need to set gateway. However NetworkManager GUI would not allow you to make a profile without a GW. You can just put the address of the other computer there or you can put 0.0.0.0 as GW.

Problem with the ethernet card - weird MAC address

I do not know how, but part of the problem was with the Realtek ethernet card I had. For some unknowable reason the computer was booting each time with slightly different MAC address and most of it was fff - like ff:ff:ff:7f:ff:ff. It was frustrating as hell as I saw in logs that IP address assigning fails. I could not solve it by trying to assign a different MAC address to it at boot. This problem was solved buying another cheap used ethernet card that seemed to come up with proper MAC address every time.

Problem on laptop end - no interface after disconnect

I found that sometimes my manjaro laptop could not see the network interface after cable was unplugged and then plugged in again. So the command mii-tool -w enp3s0 would complain that there was no such interface. However that was easy to solve by putting laptop to sleep and then waking it up again. mii-tool would report link and then ping would work as expected.

I had 2 main problems.

1. The major problem was that network manager was not completely disabled - this really messed up my configuration on A. Until I got this sorted there was no connectivity whatsoever. As soon as I disabled with mask (read about three levels of switching-off systemd) I got the connection. Both Arch and Manjaro pages about setting up static IP with netctl mention that you need to disable NetworkManager, but they don't quite say that some services may still bring NM up even though I disabled it at the 2nd level initially. Commands are:

   systemctl mask NetworkManager systemctl mask NetworkManager-dispatcher

2. Then I got the problem that sometimes this configuration would mysteriously stop working - i.e. plugging in the cable and setting laptop B NetworkManager profile to the static IP would give no results. Strangely the workstation A card link light is still on. Since at home with RaspberryPis this always works, I think this is some old hardware problems for the workstation. The best way to deal with this I found was to use mii-tool -w enp3s0 on the laptop. It means to watch your particular ethernet interface (enp3s0 in my case) for hardware level link. It changes instantly as you plug / disconnect your cable. ethtool seems more powerful but I did not find such convenient monitoring of the link for ethtool. So if mii-tool show the link, you look if your laptop interface is associated with the correct profile and then you can check with ping.

3. The third problem of course is that I don't do this kind of thing often - hence as an amateur I have many things to doubt, which makes the process more complicated. For that reason I will put some more bits here that I found particularly helpful:

One is lshw -class network -businfo - it gives you PCI address, device name and description. So on a machine with several interfaces you can make sure that your netctl or other CLI configuration is referring to the right card.

About IP v4 configuration. As you are setting up just 2 computers on the same subnet you do not need to set gateway. However NetworkManager GUI would not allow you to make a profile without a GW. You can just put the address of the other computer there.

I had 2 main problems.

1. The major problem was that network manager was not completely disabled - this really messed up my configuration on A. Until I got this sorted there was no connectivity whatsoever. As soon as I disabled with mask (read about three levels of switching-off systemd) I got the connection. Both Arch and Manjaro pages about setting up static IP with netctl mention that you need to disable NetworkManager, but they don't quite say that some services may still bring NM up even though I disabled it at the 2nd level initially. Commands are:

   systemctl mask NetworkManager systemctl mask NetworkManager-dispatcher

2. Then I got the problem that sometimes this configuration would mysteriously stop working - i.e. plugging in the cable and setting laptop B NetworkManager profile to the static IP would give no results. Strangely the workstation A card link light is still on. Since at home with RaspberryPis this always works, I think this is some old hardware problems for the workstation. The best way to deal with this I found was to use mii-tool -w enp3s0 on the laptop. It means to watch your particular ethernet interface (enp3s0 in my case) for hardware level link. It changes instantly as you plug / disconnect your cable. ethtool seems more powerful but I did not find such convenient monitoring of the link for ethtool. So if mii-tool show the link, you look if your laptop interface is associated with the correct profile and then you can check with ping.

3. The third problem of course is that I don't do this kind of thing often - hence as an amateur I have many things to doubt, which makes the process more complicated. For that reason I will put some more bits here that I found particularly helpful:

One is lshw -class network -businfo - it gives you PCI address, device name and description. So on a machine with several interfaces you can make sure that your netctl or other CLI configuration is referring to the right card.

About IP v4 configuration. As you are setting up just 2 computers on the same subnet you do not need to set gateway. However NetworkManager GUI would not allow you to make a profile without a GW. You can just put the address of the other computer there or you can put 0.0.0.0 as GW.

Problem with the ethernet card

I do not know how, but part of the problem was with the Realtek ethernet card I had. For some unknowable reason the computer was booting each time with slightly different MAC address and most of it was fff - like ff:ff:ff:7f:ff:ff. It was frustrating as hell as I saw in logs that IP address assigning fails. I could not solve it by trying to assign a different MAC address to it at boot. This problem was solved buying another cheap used ethernet card that seemed to come up with proper MAC address every time.

Problem on laptop end

I found that sometimes my manjaro laptop could not see the network interface after cable was unplugged and then plugged in again. So the command mii-tool -w enp3s0 would complain that there was no such interface. However that was easy to solve by putting laptop to sleep and then waking it up again. mii-tool would report link and then ping would work as expected.