# ifconfig -a ed0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500 inet 192.168.1.40 netmask 0xffffff00 broadcast 192.168.1.255
# ifconfig ed0 alias 192.168.1.41 netmask 255.255.255.0 broadcast 192.168.1.255
ifconfig: ioctl (SIOCAIFADDR): File exists
[using same netmask as real interface]
However, THIS WORKED:
# ifconfig ed0 alias 192.168.1.41 netmask 255.255.255.255 broadcast 192.168.1.255
[using a netmask of 255.255.255.255]
QUESTION: Why do I use a different netmask (255.255.255.255) for a IP alias on FreeBSD? Why isn't 255.255.255.0 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 192.168.1.41 is on the same subnet as 126.96.36.199 you must use 0xffffffff (aka 255.255.255.255) 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 192.168.2.40 netmask 255.255.255.0 broadcast 192.168.2.255
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