Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!tut.cis.ohio-state.edu!ucbvax!VENERA.ISI.EDU!braden
From: braden@VENERA.ISI.EDU
Newsgroups: comp.protocols.tcp-ip
Subject: Re: subnet mask problem
Message-ID: <8905261750.AA05805@braden.isi.edu>
Date: 26 May 89 17:50:59 GMT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The Internet
Lines: 23



	For instance, a mask reply on a class C network of 101.214.1.77 is obviously
	bogus. Is there an algorithm for determining whether to trust a mask response?

...
	Bill VerSteeg

Bill,

The IETF Host Requirements Working Group pondered this problem, and
here is the best we could come up with (from Section 3.2.2.9 of the
Host Requirements/Communication Layers RFC draft):

    It is recommended that the host make the following "sanity
    check" on any address mask it installs: the mask MUST NOT be
    all 1 bits, and it MUST be either zero or else the 8 highest-
    order bits MUST be on.
    
Your example would in fact fail this check, but lots of bogus
masks would pass.  We too would like to know of any better ideas.

Bob Braden