bstreile@wellfleet.com (Bill Streilein) (06/13/90)
Hello,
Mogul & Postel [RFC 950] specify that the subnet field must
never be all 0's or all 1's. Thus,
128.10.0.0 with a mask of 255.255.255.0
would be an invalid subnet. Has the thought on this changed
since this was proposed? For instance, if all broadcasts are
all 1's only, is a 'zero' subnet allowed?
Thanx in advance,
Bill Streilein
bstreile@wellfleet.com
Wellfleet Communications, Inc.romkey@asylum.sf.ca.us (John Romkey) (06/13/90)
There are a couple of reasons why all 0's is a bad idea. One is historical reasons; old versions of the BSD code used to use all 0's as the broadcast address. Another reason is to help screen out bugs. An uninitialized value will often default to 0, so from a technical perspective it's often a good idea to avoid using 0 as a magic number, like a subnet number or such. - john romkey USENET/UUCP: romkey@asylum.sf.ca.us Internet: romkey@ftp.com "There is no loyalty except loyalty to the party. There is no love except love of Big Brother. All competing pleasures we will destroy." - 1984 (film)
mogul@wrl.dec.com (Jeffrey Mogul) (06/13/90)
In article <9006121854.AA01650@rp.Wellfleet.Com> bstreile@wellfleet.com (Bill Streilein) writes: >Mogul & Postel [RFC 950] specify that the subnet field must >never be all 0's or all 1's. Thus, > > 128.10.0.0 with a mask of 255.255.255.0 > >would be an invalid subnet. Has the thought on this changed >since this was proposed? For instance, if all broadcasts are >all 1's only, is a 'zero' subnet allowed? We recommended against "subnet number 0" because there is some fear that certain software may use "0" internally to mean something like "reserved" or "not yet set". It seemed safer to ban the use of this value for numbering a real subnet, to avoid any mysterious bugs. Also, I'm not sure everyone has stopped using all-zeros broadcasts. In my experience, it is possible to get away with a subnet numbered zero, but why tempt fate? -Jeff