Windows Multiple IP Addresses and Default Gateway

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I wanted to write this post in regards to anyone currently using or admins that are about to use Microsoft Windows Servers or workstations with multiple ip addresses and how Windows will determine your preferred source address.

In this example say we have a Windows 2008 Server with 3 IP addresses:

  1. 192.168.1.10
  2. 192.168.1.20
  3. 192.168.1.30

Now if the default gateway is 192.168.1.1, the Windows 2008 Server will use 192.168.1.10 as a preferred source address, this is true as it is the IP address closest to the default gateway.

If the default gateway is 192.168.1.254, the Windows 2008 Server will use 192.168.1.30 as a preferred source address, as 192.168.1.30 is the closest IP address to the default gateway.

You can see what is being used as the preferred source address by typing in route print in the command prompt.

So even if in your TCP/IP address settings you have an IP address listed as primary, it may not necessarily be the source IP address when trying to route.

Keep the above rule in mind especially when setting up firewall rules.

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