assuming you have a class b ip address say:

150.112.0.0

With a subnet mask of
255.255.255.128

How would you work the network/host numbers?

I understand you'd be using 9 bits so you'd have a total of 510 networks, each with 126 hosts. But how would you start to list the network ID's/Host ID's? Right?

Would you assume that you used 128 as your smallest bit?

In which case it would be something close to:

Network 150.112.1.0 with hosts of
150.112.1.1 - 150.112.1.127 Broadcast 150.112.1.128?

If that's the case What would your 2nd network be? Sorry to be bothersome, just something that popped in my head that I thought would have an interesting answer. Thanks for your time!

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quote:

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Originally posted by Digs:
...
Would you assume that you used 128 as your smallest bit?
...
Network 150.112.1.0 with hosts of
150.112.1.1 - 150.112.1.127 Broadcast 150.112.1.128?
...

---

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What to happened to the first two available subnets?

# 150.112.0.1 - 150.112.0.126
Broadcast 150.112.0.127

# 150.112.0.129 - 150.112.0.254
Broadcast 150.112.0.255

I know people will suggest "Do not use all ones or all zeros subnets", but my Cisco kit is configured with 'ip subnet zero'.

However, suppose this type of question comes up in the exam. Do I assume that all ones and all zeros subnets should be avoided? Whadya think?

Cheers
Hippo

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Not all routing equipment supports subnrt 0 and 128. in a cisco only environment you could use it - so i suppose that is why it is on the books.

Golgi

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Thanks again for the info I realize that it's not a bit and actually a decimal number. My brain kinda burped and spat out bit as I've been associating the 2^x with specific # of bits. *sigh* Glad I can communicate so well! *blech*

Anyway, Thanks again, greatly appreciate the help. Crystal clear now!

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quote:

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Originally posted by golgi:
Not all routing equipment supports subnrt 0 and 128.

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Well to be honest neither have i, but lamme gives it as a possible problem on legacy equipment.

Maybe somewhere, out there in the great mesh of networks, there is a network that is wondering its being asked to route to 0 and 128

Golgi

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Hi golgi,

I'm afraid there's some confusion about what legacy equipment does not support when it comes to subnetting. Consider a class C network: 192.168.11.0. The important pieces are the net address and the broadcast address (we don't really care about the host range now):

net address: 192.168.11.0
broadcast: 192.168.11.255

Now, you want to subnet this into four subnets. Your mask is 255.255.255.192. The four subnets you get are:

- 192.168.11.0 broadcast 192.168.11.63
- 192.168.11.64 broadcast 192.168.11.127
- 192.168.11.128 broadcast 192.168.11.191
- 192.168.11.192 broadcast 192.168.11.255

Notice that the network address for the first subnet (192.168.11.0) and the network address for the original network (192.168.11.0) are identical. Also notice that the broadcast address for the last subnet (192.168.11.255) and the broadcast address for the original network (192.168.11.255) are identical. This is a potential source of confusion for legacy equipment.

Practically, in all modern networks this is not a problem anymore (the "ip subnet-zero" being on by default on IOS 12.x is Cisco's way of acknowledging this). However, from what I've seen so far, the CCNA exam wants people to be aware of the problem.

HTH

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