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Date: Tue, 06 Apr 2004 14:24:09 +0200
From: Anders Lowinger <anders.lowinger@packetfront.com>
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To: Andre Oppermann <andre@freebsd.org>
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cc: Luigi Rizzo <rizzo@icir.org>
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Subject: Re: do we support non contiguous netmasks ?
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Andre Oppermann wrote:

>>   interface ethernet 0
>>    ip address 192.168.0.0 mask 255.255.253.0
> 
> This is simply a supernet (aka classless) but *not* a non-contignous
> netmask.  A non-contignous netmask would look like 255.254.255.0.

Nope, 255.255.253.0 binary is 11111111.11111111.11111101.00000000
which is non-contignous.

>>   interface ethernet 0
>>    ip address 192.168.0.0 mask 255.255.255.0
>>    ip address 192.168.2.0 mask 255.255.255.0 secondary
>>
>>which gives the same functionality with contigious netmasks.
> 
> Not really.

Agree, not exactly the same

 > With the your second example hosts on the network have
> to have different default gateways (192.168.0.1 and 192.168.2.1)
> depending in which network range they are.  In your first example
> you just have one default gateway for all of them.  However the
> netmask has to match on all hosts otherwise you run into all sorts
> of wierd trouble.

In this case, the above is normally only used during a migration
phase (as I mentioned, this is the only use of non-contignous i've
seen, joining two separate subnets), so the hosts already have the
correct default-route in their subnet. Hosts could optionally then
be migrated to a common subnet.

/Anders