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Date: Wed, 11 Feb 2015 10:38:11 -0700 (MST)
From: Warren Block <wblock@wonkity.com>
To: Matt Churchyard <matt.churchyard@userve.net>
Subject: Re: Invalid subnet masks
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On Wed, 11 Feb 2015, Matt Churchyard wrote:

> Just been helping someone on the forums who appears to have configured their network interface incorrectly. It looks like they've assigned 250.250.250.0 as the netmask.
> I've tried assigning this netmask on a 10.1 machine and ifconfig happily accepts it.

As a tangent to that question, ifconfig(8) and the rc.conf settings do 
accept CIDR notation, which is both shorter and clearer than a dotted 
quad IP address and a separate netmask:

ifconfig em0 inet 192.168.1.1/24