[lug] CIDR question

[Carl Wagner]

My guess is:
  /24  = x.x.x.0 - x.x.x.255
  /25  = x.x.x.0 - x.x.x.128  or x.x.x.128 - x.x.x.256
  /26  = x.x.x.0 - x.x.x.63   or    64-127   or 128-191  or 192-255

assuming a /26  and a example ip address of 192.168.1.5

0000 = 0
0001 = 1
0010 = 2
0011 = 3
0100 = 4
0101 = 5
0110 = 6
0111 = 7
1000 = 8
1001 = 9 
1010 = a
1011 = b
1100 = c
1101 = d
1110 = e
1111 = f

The netmask should be 26 high order bits turned on 
  11111111 1111111 11111111 11000000 = ff ff ff c0 = 255.255.255.192
The broadcast address should be the IP address or()-ed with the not() 
of the netmask.
  ex netmask = ff ff ff c0, broadcast mask = 00 00 00 3f  
       { not(ff ff ff c0) = 00 00 00 3f }
  broadcast = IP or-ed with broadcast mask,

 11111111 11111111 11111111 11000000 = ff.ff.ff.c0 = 255.255.255.192   netmask
 00000000 00000000 00000000 00111111 = 00.00.00.3f = 0.0.0.63         
broadcast mask (my term for this)

looking at 192.168.1.5 and since it is a class c, just the last digit is
relevant.
        
         0000 0101 = 05  or just 5  (the last digit)
    or() 0011 1111 = 3f  or 63 decimal  (the broadcast mask)
        -----------
       = 0011 1111 = 3f  or 63      (the last digit of the broadcast address)
which gives a broadcast address of 192.168.1.63  

So you have:
  192.168.1.5       IP
  255.255.255.192   netmask (/26)
  192.168.1.63      broadcast address
   


Caution - this could be 100% wrong and is just my understanding of how it
works.  
I assume that if it is incorrect that others will correct the bad information.

Carl.

Andrew Diederich wrote:
> 
> Ok, I've got an easy one for someone who knows the math.  I'm switching
> ISP's today and am losing my class C for a partial C.  (Boo! hiss!)  For
> routing purposes it looks like I have two contiguous /26 networks.  From a
> fancy cidr HOWTO like thing (http://www.ibm.net.il/~hank/cidr.html) that
> tells me I have 2 one-quarter segments of a class C.
> 
> So, if I have the front end of a C (x.x.x.0 - x.x.x.128?) What would be my
> network and broadcast numbers for the two /26 networks?  I'd really like to
> find the formulas while I'm at it, 'cause it's embarrassing to ask this sort
> of question.  *grin*
> 
> -Andrew
> "Give me ambiguity or give me something else."
> 
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