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Date: Wed, 17 Sep 2003 15:33:43 -0500
From: Mark Johnston <mjohnston@skyweb.ca>
To: Adam Newhard <atnewhard@microstrain.com>
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Adam Newhard <atnewhard@microstrain.com> wrote:

> ifconfig_fxp0="inet 10.1.1.1 netmask 255.255.255.0"
> ifconfig_fxp0_alias0="inet 10.1.1.2 netmask 255.255.255.255"
> ifconfig_fxp0_alias1="inet 10.1.1.3 netmask 255.255.255.255"
> ifconfig_fxp0_alias2="inet 10.1.1.4 netmask 255.255.255.255"
> ifconfig_fxp0_alias3="inet 10.1.1.5 netmask 255.255.255.255"
> ifconfig_fxp0_alias4="inet 202.0.75.17 netmask 255.255.255.240"
> ifconfig_fxp0_alias5="inet 202.0.75.18 netmask 255.255.255.255"
> ifconfig_fxp0_alias6="inet 202.0.75.19 netmask 255.255.255.255"
> ifconfig_fxp0_alias7="inet 202.0.75.20 netmask 255.255.255.255"

> If you're configuring your system how you said above, those last 3 lines
> needs a netmask of 255.255.255.240, not .255

Actually, the lines are correct as shown.  When a second IP address is
added on a subnet where we already have one, the netmask has to be all
1's as above.  Trying to configure a second address with the same netmask
as the first results in this wonderfully specific error:

ifconfig: ioctl (SIOCAIFADDR): File exists

I believe this was the subject of hot debate and much mail to stable@
when the restriction was first introduced (4.5? 4.7?)

Mark