Question about subnetting /24's

Fri Mar 19 09:00:02 UTC 1999

Since I'm up I might as well reply...

In the preCIDR world (as I recall) the use of the all zeros and all ones 
subnet was frowned on.  When VLSM and CIDR became the norm this sorta
stopped.  It was more a fact that it is the largest waste of IPs known.
However saying that many (including CISCO) still have config rules
stopping it by default.  So in other words ya need to turn the knob.
Don't use Bay but you would think the support folks would clearly know
about this.... then again....

Chris MacFarlane
Manager, Network Operations
ACC Telenterprises

On 18 Mar 1999, Scott Gifford wrote:

> 
> OK, so this is probably somewhat of a stupid question.  But I thought
> I knew this stuff, and so did the tech support guy I almost ended up
> in a screaming match with.  :)  Can anybody let me know if I'm missing
> something I should know by now?
> 
> I have a /24 network I want to subnet into two /25's.  For realism's
> sake, let's call the network
> 
> 	216.40.155.0/24   (mask 255.255.255.0)
> 
> So I have two 126-host networks, namely
> 
> 	216.40.155.0/25   (mask 255.255.255.128)
> 	216.40.155.128/25 (mask 255.255.255.128)
> 
> The person with whom I was calmly discussing the matter said that this
> configuration didn't make any sense, because my first subnet had all
> zeroes in the last octet, so it was an all-zeroes subnet; and the
> second had all ones on the last octet, so it was an all-ones subnet.
> And that these subnets were reserved as the network subnet, and the
> broadcast subnet, much as all-zeroes and all-ones in the host portion
> indicate the network and broadcast addresses.
> 
> I would have dismissed this out of hand, but our router wouldn't let
> us use this first network unless I enabled the "zero subnet enable"
> option.  And with the second, the customer had to say it was an
> all-ones subnet or some such thing.  This seemed to imply that maybe I
> was doing something weird.
> 
> Both myself and the customer are using Bay Networks routers.
> 
> Can anybody shed any light on this?  Is this just something I've
> competely missed in everything I've learned about IP?  Are my routers
> crazy?  Is the tech support guy crazy?  Is it just that I've finally
> gone crazy?...
> 
>   Oh, and while I'm on the subject, is there a cleaner way to delegate
> IN-ADDR.ARPA for subnets of a /24 than 128 seperate delegations, one
> for each individual IP address we've assigned?
> 
>   If anybody else is interested in this, let me know, and I'll
> summarize direct responses to the list.
> 
> Thanks for any insight, suggestions, ideas, or referrals to a good
> psychiatrist in the Greater Flint area,
> 
> ------Scott.
> 
> 

Chris MacFarlane
Manager, Network Operations
ACC Telenterprises
(416) 213-2023
(416) 715-5942