This may be related to PR 228929, but it also may not. consider this sequence of events: ifconfig lo0 alias 192.168.1.1/32 ifconfig lo0 192.168.1.2/32 ifconfig lo0 192.168.1.3/32 ... now if lo0 is configured as per normal, the 2nd command will delete the address 127.0.0.1 (sorta expected) and the 3rd command will delete 192.168.1.1. In general, I would characterize the actual operation of the "alias" word to ifconfig as follows. ifconfig always adds the address to the bottom of the list of addresses. Without ALIAS, ifconfig also subtracts the top address. That is, to be clear. Using alias means there is +1 ip addresses, not using alias means that there is the same number of addresses. The top address is always the one to go away and the new address is added to the bottom. AFAICT, this behavior is not described anywhere. Additionally, as a doc bug, what ifconfig's man page says about alias --- about a non-conflicting netmask --- is no longer true.
Take.