Re: Netmask Question

Daryl Banttari ( )
Tue, 06 May 1997 11:26:21 -0500


You CANNOT use in this case since the low subnet is not a
multiple of two. The subnet mask is a bit mask: if you look at the
third octet of your subnets bitwise they are 00001011 and 00001100.
Since changes occur on bits 6, 7, and 8 of that byte, you cannot use a
11111110 subnet mask to combine them (that mask only allows host address
changes in bit 8). If your subnet numbers were and, you could supernet them together since the only bit that
changes (12=00001100; 13=00001101) is the eighth bit. This is an
oversimplification, but I'm not prepared to tech a course in binary
arithmetic here :-)

With your 2514, you can multihome so that the router will place both
subnets on the same wire (and have two addresses on the interface) but
that presents some interesting DHCP challenges, as I presume you've
noticed. If you have an Ethernet switch you could do some interesting
packet filtering to allow a single MS or Novell DHCP server grant
addresses out of both subnets (by placing two NICs in the server and
directing IP broadcasts from two sets of switch ports to exactly one
interface in the DHCP server).. but... again, I exceed the scope of this

What exactly is wrong with the status quo?


Mark A. Knight wrote:
> I am using a Cisco 2514 as my default router, but, I do have IP addresses from both Class Cs in on the same subnet. Does that mean I should use the netmask of as suggested by others. I'm not really clear what you where saying.
> Mark A. Knight wrote:
> >
> > This is a basic IP netmask question, but, possibly the answer to a few of the problems I am currently experiencing. If you have 2 or more Class C addresses (e.g. & on the same network, what should the netmask be?
> >,

-- +|Daryl S. Banttari, CNE|||"Talk does not cook rice" - Chinese proverb|'Good things come to those who wait, but only the things left by | those who hustle.' - Abe Lincoln|'A candle loses nothing by lighting another candle' - Fr. James Keller|'There is a diminishing return on caution' - Me+