Re: [lwip-users] OT: Is the AutoIP range a special case IP address range?

[David Empson]

Bill Auerbach wrote:

I just found out through testing that the PC with a NIC in the 192.168/16 subnet (netmask 255.255.255.0) will ping and connect to devices on the physical network in the AutoIP range (169.254/16).  This is on a WinXP PC.  I find the ARP table contains AutoIP addresses and I can ping and open connections to the device at the AutoIP address.

Is this something that is expected to occur?  Is it specific to Windows?  I’ve not come across any RFC that mentions that this ‘bridging’ should or can occur.

Windows has simply set up two IP addresses on the same network interface, one AutoIP and the other in the 192.168.0/24 range. If you have a look at the network traffic with Wireshark you will find that pings to 169.254/16 are being sent from the PC using an IP address in that range. No "bridging" required. The AutoIP address might not appear in "ipconfig /all", so the only evidence will be if you try to send to an AutoIP address.

It might allocate an AutoIP address when the network interface goes active (before getting a DHCP address, or even if it has a static IP address configured), and keep it for later use, or it might allocate an AutoIP address when it needs to use one because it wants to send to an address in that range.

I'm pretty sure my Mac does the same but haven't tested it lately.