IP Subnetting Question

[thomaswoo]

Hi all,

If I have a class C address, say, 192.168.1.0, and the subnet mask is 255.255.255.192. Is that true that I can only use 2 subnet (i.e. 192.168.1.64 and 192.168.1.128) ? Any problem if I use the IPs in the subnet 192.168.1.0 and 192.168.1.192 ?

Thanks in advance !

Thomas.

 

[hnd]

Your netmask allows you 4 subnets:
192.168.1. 1-62, 65-126,129-190 and 193-254

Consider the Adresses 63,127,191,and 255 are subnet Broadcast Adresses and not allowed as regular address.

Also the Adresses with all host Part bits set to 0 are illegal. (0,64,128,192)

hnd
hasso55@yahoo.com

 

[wybnormal]

In the world of Cisco you CAN use subnet 0 as a legal address PROVIDING everything on the wire understands it's "legal" Personally I turn it off since it can cause no end of troubles in a mixed network.

Mike S
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin

 

[hnd]

I know address 0 works with cisco. But that does not mean it is really legal in the TCP/IP world.
From my experience I can say: A mixed Network is lot of sh.. because the small advantage of an additional address does not equibalance the amount of trouble because of a malfunction anywhere in the network. (Address conflicts, bad routings etc.)

hnd
hasso55@yahoo.com

 

[wybnormal]

hnd- no disagreement on the mixed network I have a major mess on my hands due to someone in the past being somewhat "creative" in his solutions to networking problems.. I was just pointing out that subnet zero as "illegal" is not an absolute in networking.. should be but it's not.

Mike S
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin

 

[thomaswoo]

Thanks hnd and wybnormal,

I also agree that I can have 4 subnets to use .. however, the reason that I post this question here is because I have read 3 books (Sybex CCNA, 1 is for MCSE-TCP/IP exam and the another one is just for TCP/IP) .. and all these books said that I can only use 2 subnets .. that means I the subnet ID cannot be all 0's and all 1's ... a little bit confusing... so just want to know whether there is any "interesting" things inside the subnetting .....

Thomas.

 

[wybnormal]

This brings back a lesson I learned while getting my CNE years ago. I was struggling with the tests because I *knew* certain question's answers were wrong. My instuctor pointed out that there was the right, the wrong and the RED answer and on the tests, you must answer the RED way for the test even if you know it's wrong.

Officially speaking, you lose two subnets as illegel in the world of IP. So on the test, unless it specifically ASKS about subnet zero, answer the "color of choice" way. If you have 16 subnets.. subtract two and give the 14 as the answer. I passed my CCNA with a 945 using this method ( one of a few) I answered my MCSE IP exam the same way.

Mike S
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin

 

[hnd]

You should understand subnetting in following Way:

A TCP/IP Address consist of 32 Bit: The network Part consist of n subsequent bit set to 1 the host part of 32-n subsequent bit with value 0.(n<1<32)
That is the netmask.

There is one must: A host adress with all bits set are not allowed. A Host address with all bits reset should not be used.

Therefore if you are not connecting to internet nearly each netmask would be possible. If a netmask of 8.0.0.0 would make sense is an other question.


hnd
hasso55@yahoo.com

 

[Guest_imported]

All true, but the key I believe is the way differing routers deal with all 1s or 0s in the subnet ID. Many esp. older routers don't like it.
Troy

 

[hnd]

@TroyW

Be aware that most routers (Even newer routers) do not support class D Networks (class D: Network Part >224.0.0.n)

hnd
hasso55@yahoo.com

 

[wybnormal]

I find that comment interesting.. Cisco most certainly does suport Class D as it's the multicast range. As do many of Cisco's switches. I can not address other vendors equipement aside from 3com( which I'm ripping out) My limits are Cisco-centric.

There also OSPF extensions for multicast routing, it's used for MBONE and some other applications. It is something of a bother to setup due to having groups and whatnot

Mike S
&quot;Diplomacy; the art of saying 'nice doggie' till you can find a rock&quot; Wynn Catlin