[CLUE-Tech] WAS: Netmask trivia question...

[black at galaxy.silvren.com]

I think what Perlman was referring to is that when IP was first conceived,
addresses were all classful. VLSM didn't exist, and you could assume a
netmask based on the host address.

Since VLSM, all that goes out the window and to support it you must have
contiguous ones in the netmask, since you can no longer assume the mask
length in classless addressing.

Interestingly that book was published in 1999, when a lot of people were
still cutting their teeth trying to move from classful to classless
addressing, and the IP address space shortage was all the rage.

If Perlman was referring to the time before VLSM, then I concede that you
are correct!


On Sun, 31 Aug 2003, Keith Hellman wrote:

> On Sun, Aug 31, 2003 at 12:37:16PM -0400, black at galaxy.silvren.com wrote:
> > Hi, not to start stepping on toes, but netmasks quite positively have to
> > be contiguous 1's. 255.255.1.0 and the like are impossible. You can't
> > borrow bits for host addresses arbitrarily.
> >
> > 255.255.1.0 is a valid HOST address, but it is most certainly not a valid
> > netmask.
>
> And I quoth:
>   "In theory, IP allows the mask to have noncontiguous 1's.  This means
>   that the bits that indicate the link number need not be contiguous but
>   can be sprinkled thoughout the address."
>
>   Interconnections (Bridges, Routers, Switches, and Internetworking
>   Protocols), 2nd Edition.  Radia Perlman, Addison-Wesley, ISBN
>   0-201-63448-1  p. 195
>
> Admittedly, she goes on to point out that it is ill-advised to use
> non-contiguous netmasks:
>
>   "...and in fact none of the router vendors supports noncontiguous
>   subnet masks, so we really can think of the link address as a prefix."
>
> Radia Perlman, IIRC, is the lady that developed the layer 2 bridging
> algorithm, and is still actively involved in network research.  I highly
> recommend the book BTW.
>
> --
> Keith Hellman                             #include <disclaimer.h>
> khellman at mcprogramming.com                from disclaimer import standard
>
> "You want to know the secret to immortality, write a bunch of songs that people
> keep singin' and playin'."
>
> -- Dr(x) John on Duke Ellington
>
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