[e2e] Addressing format...network and mask

Joe Touch touch at ISI.EDU
Mon Dec 1 09:29:11 PST 2003

Michael B Greenwald wrote:

>    Mon, 1 Dec 2003 13:51:47 +0530
>    "Alok Dube" <alok.dube at apara.com>
>    I need a way to propogate the 1st address and last address of a network.
>    Incase i have 129 hosts in a network, i simply should propogate the 1st
>    address and the last address (or the number of hosts) instead of the mask.
> I'm not sure what you really need to do here, but if I understand
> somewhat correctly, then why not just allocate two subnets, one with
> 128 hosts and one with 1 host?  If you are trying for a dense
> allocation of addresses within the net, then, in the absolute worst
> case, there will be 25% waste.  Usually, it will be much more densely
> packed (no waste at all in your example of 129).

/25 subnets can't support 128 hosts; the max is 125. The result would be:

1 /25 subnet with 125 hosts
	all 0's depricated
	all 1's broadcast
	one address for the router

1 /29 subnet with 3 hosts
	4 hosts
	1 router
	all 0's
	all 1's
	1 unused

That's 129/130 max possible (99.2% efficient use of host addresses), 
though it's only 94.9% efficient with the space as a whole.

However, if you've got 129 hosts and you're using a /24, you're not 
doing badly at all.

The first address/last address method isn't as useful because it doesn't 
communicate the routing information, i.e., first and last of the 129 
could be a (/25, /29) pair, or a single (/24), or many other combinations.


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