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Re: traffic filtering

  • From: E.B. Dreger
  • Date: Tue Jan 22 12:07:51 2002

> Date: Tue, 22 Jan 2002 11:48:52 -0500 (EST)
> From: Stephen Griffin <stephen.griffin@rcn.com>

> In the referenced message, Walter Klomp said:
> > As far as I know .0 and .255 are network and broadcast addresses
> > respectively, NEVER should a workstation be configured on these addresses,
> > unless something drastically changed in the RFC's for IPv4 which I am not
> > aware of...

CIDR

> only on a /24. on /0 - /23 only the first .0 is network, and the last
> .255 broadcast. on /25-/30 it depends on where the network begins and
> ends. /31 has no directed broadcast. /32 is a single host and similarly
> has no directed broadcast.

Or, put another way:  Do the addresses in binary.  Then convert
to dotted quad.

> > I for one am filtering .0 and .255 at my border routers, and also rate
> > limiting echo at a reasonable rate... and have never gotten a complaint
> > about people not being able to reach or be reached...

Ughh.  Take 10.0.0.0/22: What is 10.0.0.255? How about 10.0.1.0?

Misconfiguration like this is why I (and others) recommend not
using ...0 or ...255 addresses, even if valid.

As you (Stephen) pointed out, what about 172.16.16.16/29?  The
smurf amplifiers there would be 172.16.16.16 and 172.16.16.23.
In incomplete C:

	uint32_t ip_addr ;
	uint32_t netmask ; /* assume that it's valid */

	if ( 0 == (ip_addr & ~netmask) )
		this_is_all_0s ;

	if ( ~netmask == (ip_addr & ~netmask) )
		this_is_all_1s ;


Eddy

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