Re: 255.255.255.129 netmask?

Michael H. Warfield (mhw@wittsend.com)
Fri, 25 Sep 1998 09:28:38 -0400 (EDT)

Samuel Leo enscribed thusly:
> > I recently got cable Internet access and my provider says my subnet mask
> > needs to be 255.255.255.129. Being a security-conscious kind of guy, I

> It must be a typo, try 255.255.255.128.

Actually... I'm betting it should have been 255.255.255.192. That
would be a simple transposition of the two digits and 192 is a legitimate
contiguous mask number (which 129 is not). That would be a /26 subnet and
would make sense and provide space for 62 address (64 minus the all zeros
and all ones addresses). The .128 would be a /25 subnet and would also
make sense and would be a bit broader (126 addresses)...

255.255.255.129 works out to be a mask like this:

11111111 11111111 11111111 10000001

While "technically" some of the RFC's DID allow for non-continuous
subnet masks, I know of very few systems that support them. For all
practicality (theoretical RFC acceptance aside) this was not a valid subnet.

255.255.255.192 works out to be a mask like this:

11111111 11111111 11111111 11000000

That's actually only a single bit different that the .128 mask
suggested by this previous responder. Either are valid and both are
pretty close. I would get back to the cable dudes and find out if one
of them was temporarily dislexic...

Mike

-- 
 Michael H. Warfield    |  (770) 985-6132   |  mhw@WittsEnd.com
  (The Mad Wizard)      |  (770) 925-8248   |  http://www.wittsend.com/mhw/
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