Valid subnets.
 

Hello guys, I am currently getting ready for the ccna test, the course I am taking is suggesting that the ip address 192.168.55.1/30 is a valid host ip, I believe it is not since the first usable host address with that netmask will be 192.168.55.5 I am correct?

thanks in advance.

this is a valid address and netmask. not sure why you think the 1st address would be nn.55.5

http://grox.net/utils/whatmask/

------------------------------------------------
TCP/IP NETWORK INFORMATION
------------------------------------------------
IP Entered = ..................: 192.168.55.1
CIDR = ........................: /30
Netmask = .....................: 255.255.255.252
Wildcard Bits = ...............: 0.0.0.3
------------------------------------------------
Network Address = .............: 192.168.55.0
Broadcast Address = ...........: 192.168.55.3
Usable IP Addresses = .........: 2
First Usable IP Address = .....: 192.168.55.1
Last Usable IP Address = ......: 192.168.55.2

Hello Blinkey, thanks for your reply, here is the logic of my post:
in binary the network address and the netmask:

11000000.10101000.00110111.00000000 -> 192.168.55.0
11111111.11111111.11111111.11111100 -> 255.255.255.252
-----------------------------------

As you can see the the first network id bit is in the 3rd position from right to left, that bit has a value of 4, since we can't use all 1 in an address neither all 0 that bit has to be on for the first valid network id that will give us an address of 192.168.55.4 for the first available network, in binary:
11000000.10101000.00110111.00000100 now the first bit for the host has to be on for the first valid host address that will give us a first valid host of 192.168.55.5 in binary:
11000000.10101000.00110111.00000101
so in the case of 192.168.55.1 11000000.10101000.00110111.00000001
will require all the network bits in that octet to be 0 which is not possible, now the problem is that the source of the course I am taking seems to be a source you can thrust, that is why I have doubts, any help will be appreciated.

Does the all zero rule apply to an octet, or the whole 32 bits? I think it is all 32 bits. Actually, it should apply to both sides of the net mask. (Network id and host id)

I thought the octet was purely used to make the address more "human" readable. Computers don't need such help.

If it is an Octet, then 192.168.0.n with a netmask of 255.255.255.0 would not be vaild. Yet this must be the most widely used address anywhere.

I also thought about the 192.168.0.n but maybe there is not a problem since there no subnetting involved.
That is what I am not sure about, check this up:
http://www.networkcert.net/network/subnetting.htm they call it "delta"

Subnetting is always involved.

The ip address is a binary number, that is 32 bits long. Don't be fooled by the human readable form where we see 4 numbers that are equivilant to a binary octet separated by dots.

The ip address in binary form has no dots.

I do not know why the site you pointed at has ignored the 131.107.0.0 subnet as this is a valid network id. They have also ignored subnet 131.107.224.0. But they only needed 6 subnets, which they already had.

------------------------------------------------
TCP/IP NETWORK INFORMATION
------------------------------------------------
IP Entered = ..................: 131.107.0.0
CIDR = ........................: /19
Netmask = .....................: 255.255.224.0
Wildcard Bits = ...............: 0.0.31.255
------------------------------------------------
Network Address = .............: 131.107.0.0
Broadcast Address = ...........: 131.107.31.255
Usable IP Addresses = .........: 8190
First Usable IP Address = .....: 131.107.0.1
Last Usable IP Address = ......: 131.107.31.254

One more comment. I have not done any Cisco certification, but I have done some MS ones. When I was studying I looked at lots of practice tests and aids on the net. Some were really good. But many were rubbish. They explained things poorly and some were just plain wrong.

I would recommend that you buy at least one "big fat book" from a reputable publisher.

Do lots of reading and practice tests.

Good luck.

PS: If my logic on the ip addressing above is wrong, please update this thread.

Hello Blinkey, sorry what I meaned by subnetting was by not using default masks, the thing is I learned how to subnet on a cbtnuggets course and some other material, that is why when I saw that network address being used in some other material it triggered my doubts, then after posting I found the link that I posted here, so far I think it is necesary to substract the 000000 and the 111111 from the pool of networks ids, since the technique I learned to subnet accounts for it and that I learned from a reputable publisher, but you know how this is, and I am not 100% sure so I was hoping someone can help. cheers

Finally, I got it here is a note on a cisco site.

This is the interesting part:

Note: In the past, there were limitations to the use of a subnet 0 (all subnet bits are set to zero) and all ones subnet (all subnet bits set to one). Some devices would not allow the use of these subnets. Cisco Systems devices will allow the use of these subnets when the ip subnet zero command is configured.

Blinkey Thanks for your help.

Hey Carlosruiz, Have a read of the first few pages of this RFC.

http://rfc.net/rfc1700.html

Like the Cisco document, shows your original thinking was right, but dated.

Hello Blinkey, thanks for the link, no doubt I got a pretty outdated course, but is good to know in case I find one of those old devices out there.

Thanks a lot for your valuable help.