Date: Sat, 16 Nov 2002 16:21:34 +0000 From: Matthew Seaman <m.seaman@infracaninophile.co.uk> To: freebsd-questions@FreeBSD.ORG Subject: Re: question on IP alias/broadcast Message-ID: <20021116162134.GB12726@happy-idiot-talk.infracaninophi> In-Reply-To: <5.2.0.9.2.20021116082511.00b26508@molson.wixb.com> References: <058f01c28d76$22296230$020aa8c0@morpheous> <5.2.0.9.2.20021116082511.00b26508@molson.wixb.com>
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On Sat, Nov 16, 2002 at 08:28:32AM -0600, J.D. Bronson wrote:
> I setup an alias statment in rc.conf and it seems to work fine..
> but I have a question on the broadcast IP:
>
>
> em0: flags=8843<UP,BROADCAST,RUNNING,SIMPLEX,MULTICAST> mtu 1500
> options=3<rxcsum,txcsum>
> inet 192.168.100.1 netmask 0xffffff00 broadcast 192.168.100.255
> inet 192.168.100.4 netmask 0xffffffff broadcast 192.168.100.4
> ether 00:a0:9d:23:0b:f6
> media: Ethernet autoselect (100baseTX <full-duplex>)
> status: active
>
>
> Why isnt the alias broadcast set to .255 ?
Because the broadcast address is actually completely determined by the
combination of the inet address and netmask. It's redundant
information really.
Remember that an IP number or netmask expressed as a dotted quad is
really just a 32bit unsigned integer, and can be equally well
expressed as a hexadecimal or even decimal integer. thus:
192.168.100.1 is the same as 0xc0a86401 or 3232261121
and
0xffffff00 is the same as 255.255.255.0 or 4294967040
Thus you can take an inet address and a netmask and bitwize logic to
generate the network address:
$network = $inet & $netmask;
and from the network address and the netmask, you can generate the
broadcast address:
$broadcast = $network | ~$netmask;
So your question really boils down to "why is the netmask 0xffffff00
for the first address and 0xffffffff for the alias address?" That's
pretty much simply so that the kernel knows which source address it
should put into outgoing packets when it isn't already determined.
Cheers,
Matthew
--
Dr Matthew J Seaman MA, D.Phil. 26 The Paddocks
Savill Way
Marlow
Tel: +44 1628 476614 Bucks., SL7 1TH UK
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