What could be the reason for a Windows System to have multiple preferred address?

Question

While googling, I came across the following Question Multiple Preferred IPv4 Addresses, where the user reported that multiple IPs were shown as preferred. I found few other places where the same problem was discussed but the reason was not explained anywhere.

Can anyone help me understand?

1. How can multiple IPs appear under preferred IP?
2. Is this unique to Windows?
3. What would be the consequence of multiple preferred IPs?

Answer 1

Having multiple preferred IPv4 addresses is normal and well-treated by the standards. The inet family provides the capability to assign multiple addresses to each logical unit, with each address equally represented on the interface.

Here are some facts :

• Any network interface can have multiple addresses on a single logical unit.
• Each logical unit can only have one primary IP address, but multiple preferred addresses.
• A primary address is the local (source) address that is used when sending packets from the computer to a destination address that is not local to any configured subnet.
• The preferred address is used when an interface has two addresses configured within the same subnet. The default selection of the preferred address is the lowest numerical prefix.
• If there is only one address, it is both primary and preferred.
• All addresses are allocated by the DHCP master or masters for the local network or networks, or are alternatively locally set as fixed in the interface.

Having multiple preferred IPv4 addresses is however not the normal situation. The usual cause is the router, for example when splitting the network into the sub-segments of main and guest subnets.

This situation can also be caused by the manual configuration of the local network, or by simple inefficiencies in the router granting several leases to the same computer, or by other local network architecture segmentation such as VPN to another computer on the same network.

If having multiple preferred IPv4 addresses was not intentional, it can normally be corrected by the release of all DHCP leases and the renewal of only one lease.

Answer 2

I'll try to keep it simple...

If you have more than 1 network card (NIC) in your PC they can be in the same subnet (same ip network and same mask, usually 192.168.1.0/24 mask(24) is 255.255.255.0 24/32 bits) If you have 2 NIC, for example, they could be 192.168.1.1/24 and 192.168.1.2/24.

The OS needs to infere a default route for the outgoing ip packets... so it decides for the lowest numeric prefix, in this case 192.168.1.1/24.

Well, it's a bit more complicated than this, but in a simple case it's this what would happened... (I had assumed the first NIC you put in the PC were 192.168.1.1 ;) )