(?) Homework question: defining subnets

From Darren Collins

Answered By Faber Fedor, Frank Rodolf, Jim Dennis

How do you specify the first 6 & last 2 available subnets defined by the IP address and subnet mask ( Any help on this would be greatly appreciated.

Sincerly, Darren

(!) [Faber] How do I? Very well, actually. I usually use a calculator, but using an abacus is much more challenging!
(!) [Frank] But doing it by hand is more fun!
(!) [Faber] Have you looked here http://www.learntosubnet.com ?
BTW, we don't do homework assignments around here, except for our own, of course.
(!) [JimD] The reason we know it's a homework assignment is because no one uses this scheme in the real world. This would define 126 subnets of 510 hosts each (or 128 subnets --- with modern equipment). The first subnets would be 155.25.0.* AND 155.25.1.*; the next would be .2.* and .3.*; etc.
This is a bit-aggregation (supernetting) over the traditional "class C" sized network. Normally people would go for a 4-bit aggregation --- giving them 16 networks of about 4000 hosts each (and possibly fanning those into class C sized networks through another hierarchical layer of routers).
(!) [Frank] Actually, there are real world exceptions to the 4-bit aggregation. The multinational I am working at nowadays uses a 21-bit mask on a lot of sites. (And it still has me confused at times.)

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Published in Issue 83 of Linux Gazette, October 2002

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