The auto-configuration IP address and its uses...?

sram

[H]ard|Gawd
Joined
Jul 30, 2007
Messages
1,469
I was asked to troubleshoot some issues in a network so I started working on it. One issue was that one network printer stopped printing. I checked connectivity and saw that RG-45 lights are properly lit so that's one thing fine. I went on to check its IP address from its control panel and it gave me something like 169.254.xxx.xxx, so I immediately told myself that the printer isn't actually really connected because that's the auto-configuration IP address. I later noticed that the network doesn't have a router so there is only a switch and every device is assigned an IP address manually (No DHCP of course). When I do a network search for printers, the printer in question appears with its 169.254.xxx.xxx ip address and I was always avoiding it but I later said let's just add it and install its driver. I did and it worked. So the printer is on 169.254.xxx.xxx and the PC is on 192.168.11.5 for example. I understand that this thing is an apipa thing but how does it work exactly?? Two devices can communicate while not in the same subnet with no routing involved?

Bear with me please. I'm having a hard time remembering the stuff in my network classes:unsure:
 

FNtastic

[H]ard|Gawd
Joined
Jul 6, 2013
Messages
1,419
I was asked to troubleshoot some issues in a network so I started working on it. One issue was that one network printer stopped printing. I checked connectivity and saw that RG-45 lights are properly lit so that's one thing fine. I went on to check its IP address from its control panel and it gave me something like 169.254.xxx.xxx, so I immediately told myself that the printer isn't actually really connected because that's the auto-configuration IP address. I later noticed that the network doesn't have a router so there is only a switch and every device is assigned an IP address manually (No DHCP of course). When I do a network search for printers, the printer in question appears with its 169.254.xxx.xxx ip address and I was always avoiding it but I later said let's just add it and install its driver. I did and it worked. So the printer is on 169.254.xxx.xxx and the PC is on 192.168.11.5 for example. I understand that this thing is an apipa thing but how does it work exactly?? Two devices can communicate while not in the same subnet with no routing involved?

Bear with me please. I'm having a hard time remembering the stuff in my network classes:unsure:
The only way for devices on different subnets to communicate is via a router. A router is the only device that can send layer 3 (IP) traffic to the right place. Everything logically says that these two devices shouldn't be able to communicate.
However, I did a lot of research and found something similar to what you found. https://answers.microsoft.com/en-us...as-apipa/2a560a3e-4d3a-4d74-9617-8b3c9fc12cf5
I'm going to speculate that because APIPA is a special range, and a printer will send out broadcasts to everyone about its service being on the network (ostensibly, not obeying the rules that we are used to as it's a special network range and may behave in an unexpected manner).
You are correct. Technically, it shouldn't work. And, for it to be correct, you should update the printer's address to an appropriate one. Arguably, everything is working though... I would suggest changing the IP to one on the same subnet so that you know the expected behavior. Because, well, that configuration is not working as expected right now. If you're comfortable with it, it's going to be your call
 

k1pp3r

[H]F Junkie
Joined
Jun 16, 2004
Messages
8,276
HA, i just went through this yesterday.

If you set a static IP in modern OS's it will check the network for duplicates, if it finds one it sets it to the 169 address unless DHCP scope is available.

Choose a different static IP, the one you chose is already in use.
 

sram

[H]ard|Gawd
Joined
Jul 30, 2007
Messages
1,469
The only way for devices on different subnets to communicate is via a router. A router is the only device that can send layer 3 (IP) traffic to the right place. Everything logically says that these two devices shouldn't be able to communicate.
However, I did a lot of research and found something similar to what you found. https://answers.microsoft.com/en-us...as-apipa/2a560a3e-4d3a-4d74-9617-8b3c9fc12cf5
I'm going to speculate that because APIPA is a special range, and a printer will send out broadcasts to everyone about its service being on the network (ostensibly, not obeying the rules that we are used to as it's a special network range and may behave in an unexpected manner).
You are correct. Technically, it shouldn't work. And, for it to be correct, you should update the printer's address to an appropriate one. Arguably, everything is working though... I would suggest changing the IP to one on the same subnet so that you know the expected behavior. Because, well, that configuration is not working as expected right now. If you're comfortable with it, it's going to be your call
Very interesting. Thanks for this input man. The whole network needs to be rebuilt actually, not only the printer IP address.....I will see what I can do.
 

Dead Parrot

2[H]4U
Joined
Mar 4, 2013
Messages
2,831
Like the OP, basic networking study is years past. Since all devices are on the same physical network with no routers/gateways, the devices could just use MAC addresses.

Might add a router or other gizmo in just a DHCP mode, not using any other functions. Would prevent this behavior when future new devices get plugged in. Use an assigned range of say 10.10.2.1-50 for known devices and 10.10.2.100-150 for new devices. A check of the DHCP addresses leased out will quickly reveal new gizmos plugged in as they are the ones in the 100-150 range. Then you can go looking for the new gizmo.
 
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