clarification on pg 142

[joh]

Just to clarify. If i had 3 networks. network A & B with blocks of 8 and network C with block of 64 that network A could be subnet .0 , network B could be .8 , but network C could not be .16, or .32, . That infact network C could only be assigned subnet .0, .64, .128, or .192. ??

[aguilera]

Ok, It is easy to get confused when all of these numbers are just thrown at you. A block size of 64 just means that you have a block of 64 IP addresses. This means your networks are counted out in 64....0, 64, 128, 192...

Network A will have a block size of 8: .0 - .7 (mask of /29)
Network B will have a block size of 8: .8 - 15 (mask of /29)
Network C will have a block size of 64: .65 .126 (mask of /26)

The next available network will now start with .128, but you still have from 16-63 avialable to use as well....so, the better bet would be to start at 0 with a block of 64, which would be valid hosts of 1-62, then for two block sizes of eight you can use subnets 64 and 72...that works better....

Hope this helps!

[joh]

The part of the question that was most important to me was on the block 64 for network C , can you assign it as subnet .32 or does it " have to be " subnet 0,64,128, or 192

[lammle]

Every block size starts at zero (assuming ip subnet-zero is on) and you must count only in increments of that block size.

For a /27, it would be 0, 32, 64, 96, 128, 160, 192, and 224.
for a /26, it would be 0, 64, 128, and 192
For a /25, it would be 0, and 128

you cannot deviate from this - in other words you cannot start wherever you want....

As you have larger block sizes, you have less subnets available.

It is critical that you understand this and study this until you have every mask and block size memorized.

Cheers!
Todd Lammle

[stringchopper]

looking to clarify the answers provided...

If I have two /27's (A & B) and one /26 subnet (C) on the 192.168.1.0 /24 network...

A could be:
192.168.1.0 /27 (hosts 1.1 - 1.30)

**C** could be:
192.168.1.32 /26 (hosts 1.33 - 1.94)

B could be:
192.168.1.96 /27 (hosts 1.97 - 1.126)

Notice the order: A,C,B (ranges 0 - 31, 32 - 95, 96 - 128)

in other words, my block sizes are all good, but a 64 block doesn't have to start at a subnet divisible by 64, as long as it truly covers a block of 64.

Correct?

[stringchopper]

dang, I just posted that, and now that doesn't seem to make sense,

If I had a 192.168.1.32 /26 subnet, that last octet would be, in binary
00 100000

which would mean it is in the 192.168.1.0 subnet, ie, 0 is the network, and 192.168.1.63 is the broadcast....

ok, i'm cool now.

:)