Mike wrote:
# ifconfig -a
        inet netmask 0xffffff00 broadcast

# ifconfig ed0 alias netmask broadcast
ifconfig: ioctl (SIOCAIFADDR): File exists
[using same netmask as real interface]

# ifconfig ed0 alias netmask broadcast
[using a netmask of]

QUESTION: Why do I use a different netmask ( for a IP alias on FreeBSD? Why isn't used?

From the ifconfig man page on 4.9

alias   Establish an additional network address for this interface.  This
        is sometimes useful when changing network numbers, and one wishes
        to accept packets addressed to the old interface.  If the address
        is on the same subnet as the first network address for this
        interface, a non-conflicting netmask must be given.  Usually
        0xffffffff is most appropriate.

Since is on the same subnet as you must use 0xffffffff (aka as the netmask. (I don't know *why* I'm afraid).

You could always stick the alias on a different subnet at which point you can use the netmask you expect.

e.g. # ifconfig ed0 alias netmask broadcast


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