About IP Addressing

Question

Ok... got a strange one... whenever I connect my laptop to any unmanaged switch with any of our machine centers it always issues a 169.254.206.112 IP Address in the IPv4 settings and I have to delete the ip address before connecting to the plc I am needing. Is there anyway to delete it permanently? This did not use to happen at all.. and only recently started.

Answer 1

That probably happens because you have your computer set for DHCP assigned addresses, and whatever port you plug into doesn't have a DHCP server anywhere to provide an address. When that happens, the operating system can see that a network is connected, but address requests aren't being satisfied, so the OS assigns what is called a "Link Local" address.

Comments

Ok... the only issue with that is.. even if I set a static IP Address instead of automatically obtaining one in the IPv4 setttings... when I connect to the switch the IPv4 still issues a 169 IP address so that now two IPv4 addresses show up.

Answer 2

Yes, your laptop's LAN is most likely configured for DHCP and your managed switches are acting as a DHCP Server and dynamically assigning an IP address to your laptop. When you connect to unmanaged switches they cannot provide the DHCP service that your laptop is requesting, so Windows is assigning an address using APIPA...

"Automatic Private IP Addressing - The Windows function that provides DHCP autoconfiguration addressing. APIPA assigns a class B IP address from 169.254.0.0 to 169.254.255.255 to the client when a DHCP server is either permanently or temporarily unavailable."

What I do is assign a Static IP/Subnet Mask address to the "Alternate Configuration" tab for my programming laptop's LAN properties. This way, when the DHCP configured "General" tab cannot "Obtain an IP address automatically", it reverts to the Static configuration on the alternate tab.

This is fine if the alternate configuration is unique to any of the networks you plug in to. Also, the configured alternate subnetting must be capable of reaching all network devices.

Failing the alternate method being suitable to your needs, you would have to assign a manual IP address on the "General" tab with "Use the following IP address", before you connect to an unmanaged switch.

Answer 3

This can happen for a whole list of reasons which I don't care to list all of them here but normally it can be resolved by either disabling APIPA on the intrface or the whole machine.

Many times the IPV6 tunneling interface can cause this issue if it's enabled and set to DHCP. if you don't use IPV6 just disable it.

Some devices out the box set their DHCP server to issue in the APIPA range and again in networking there are some valid reasons for doing this and the APIPA address will work just fine as long as all the devices are in the same /16 subnet but it will not route anywhere.

If you want to disable APIPA here is how.

This will Disable it by adaptor

Use Registry Editor to create the following registry key, where adapter name is the name of the Dynamic Host Configuration Protocol (DHCP) configured adapter where you want to disable APIPA:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\ Services\Tcpip\Parameters\Interfaces\Buchanan
Add the following value to this key:
Value name: IPAutoconfigurationEnabled
Value type: REG_DWORD
Value in hexadecimal: 0 (A value of 0 disables APIPA support on this adapter)
NOTE: If the IPAutoconfigurationEnabled entry is not present, a default value of 1 is assumed, which indicates that APIPA is enabled.

This will disable it on the entire machine.

Use Registry Editor to create the following registry key:
HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Servic es\Tcpip\Parameters
Add the following value to this key:
Value name: IPAutoconfigurationEnabled
Value type: REG_DWORD
Value in hexadecimal: 0 (A value of 0 disables APIPA support on this computer)
NOTE: If the IPAutoconfigurationEnabled entry is not present, a default value of 1 is assumed, which indicates that APIPA is enabled.

You also need to restart the machine after you do this.