Re: Ethernet is not started at boot

[<tomas@tuxteam.de>]

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On Wed, Feb 07, 2018 at 09:58:55PM +1100, Erik Christiansen wrote:
> On 07.02.18 09:33, Michelle Konzack wrote:

[...]
> > auto eth2
> > iface eth2 inet static
> >         address         192.168.4.12
> >         netmask         255.255.127.0
> >         gateway         192.168.4.1
> >         network         192.168.4.0
> > 
> > So, it is not so good idea to leafe it out
> 
> Michelle, that netmask is 0b11111111111111111000000100000000 , which is
> the first I have ever seen with a hole in it.

No, that would be 1111 1111  1111 1111  0111 1111  0000 0000 (some spacing
added for legibility). But the "hole" thing still is there. I have no idea
what network equipment will do on that. But at least the Linux networking
stack keeps you from doing that:

  tomas@trotzki:~$ sudo ifconfig eth0 192.168.230.5 netmask 255.255.127.0
  SIOCSIFNETMASK: Invalid argument

Whereas:
  tomas@trotzki:~$ sudo ifconfig eth0 192.168.230.5 netmask 255.255.128.0

does work without complaints.

(to the purists out there: I can't for the life of me remember what 'ip'
wants me to chant to achieve that! I keep using both...)


>                   For 512 addresses, I'd
> have expected 255.255.254.0, which has contiguous bits:
> 0b11111111111111111111111000000000 (If 10 bits (1024 addresses) were the
> aim, then it would be 252, but never 127, if I have any kind of grip on
> this stuff.)

Yes.

> Not trusting the software to handle an unusual netmask, I might have
> been tempted to just use a class B netmask, 255.255.0.0 , but then I can
> be a Feigling in such matters.

CIDR should Just Work. But holes, OTOH...

Cheers
- -- t
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