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## Help with understanding ip address problem

Hi

Can someone please help me with this. I enetered the following in my pix 515e ver (6.2):

But I got the following reply:

IP and subnetmask may be an invalid pair

I don't see why this came up. Can someone please explain it to me. I can't see anything wrong with the ip and subnet.

Thanks

Dan

1 ACCEPTED SOLUTION

Accepted Solutions
Community Member

## Re: Help with understanding ip address problem

Amigo, To solve that, it's very simple: (Recall CCNA subnetting course)

So --> 192 is the interesting octet in mask

1).Find the Magic Number, By Substracting:

256 - Interesting Mask octet -> 256 - 192 = 64

2) Find a Multiple of Magic Number (Here :64) which is less or equal than interestzing octet of Address (also 64).

Then First Multiple of 64 = 64 ,

Then your working subnets start from 64

1st subnet 255.255.255.64,

-> Add 64 to find the following subnets :Always < 256

2nd subnet 255.255.255.128

3rd subnet 255.255.255.192

SO YOUR AVAILABLE IP ADDRESSES in 1st subnet are From : xxx.xxx.210.65 TO : xxx.xxx.210.126

Remember xxx.xxx.210.64 and xxx.xxx.210.127 are not allowed as IP address (Subnet Number and Broadcast)

Now, if you want subnets with 8 hosts each , then you have to solve (2^y - 2 = 8) or (2 ^y = 10)

you will have y = 4

(because 2^4 = 16 and 2^3 =8 which is unsuffisant)

So you have to reserve 4 bits for subnetting.

Meaning it is 255.255.255.240

240 = 128+64+32+16 (2^7 + 2^6 + 2^5 + 2^4)

To find the available Ip addresses for /28, same reasoning than above (magic number is 16 here)

So They are FROM xxx.xxx.210.65 TO xxx.xxx.210.78

xxx.xxx.210.79 is not allowed (broadcast) and in

this case you have 14 available addresses

8 REPLIES
Silver

## Re: Help with understanding ip address problem

72=0100 1000

With 248 or /29 as a subnet mask, the host bits are all zeros which is not valid. Make it 73.

Thanks.

Green

## Re: Help with understanding ip address problem

Dan,

IP/72 - 0100 1000

SM/248 - 1111 1000

In your case all but the last 3/32 bits have to match. Last 3 bits can be either 0 or 1.

Pls. rate the post if it helped.

HTH,

Sundar

Community Member

## Re: Help with understanding ip address problem

Hi

Thanks for the replies.

I need a subnet with 8 ip's available. Could you please suggest one for me.

I have the following range:

xxx.xxx.210.64/26

Thanks

Dan

Community Member

## Re: Help with understanding ip address problem

Amigo, To solve that, it's very simple: (Recall CCNA subnetting course)

So --> 192 is the interesting octet in mask

1).Find the Magic Number, By Substracting:

256 - Interesting Mask octet -> 256 - 192 = 64

2) Find a Multiple of Magic Number (Here :64) which is less or equal than interestzing octet of Address (also 64).

Then First Multiple of 64 = 64 ,

Then your working subnets start from 64

1st subnet 255.255.255.64,

-> Add 64 to find the following subnets :Always < 256

2nd subnet 255.255.255.128

3rd subnet 255.255.255.192

SO YOUR AVAILABLE IP ADDRESSES in 1st subnet are From : xxx.xxx.210.65 TO : xxx.xxx.210.126

Remember xxx.xxx.210.64 and xxx.xxx.210.127 are not allowed as IP address (Subnet Number and Broadcast)

Now, if you want subnets with 8 hosts each , then you have to solve (2^y - 2 = 8) or (2 ^y = 10)

you will have y = 4

(because 2^4 = 16 and 2^3 =8 which is unsuffisant)

So you have to reserve 4 bits for subnetting.

Meaning it is 255.255.255.240

240 = 128+64+32+16 (2^7 + 2^6 + 2^5 + 2^4)

To find the available Ip addresses for /28, same reasoning than above (magic number is 16 here)

So They are FROM xxx.xxx.210.65 TO xxx.xxx.210.78

xxx.xxx.210.79 is not allowed (broadcast) and in

this case you have 14 available addresses

Community Member

## Re: Help with understanding ip address problem

Hi

Could you please tell me the following:

1) Why are there 4 bits reserved for subnetting?

2) Would my new command be:

"ip address dmz xxx.xxx.210.73 255.255.248.0" ?

2) How to remove the ip address command from teh pix. When I enter "no" as a prefix to the sttement teh pix says "no" not supported for "ip address".

Dan

Hall of Fame Super Gold

## Re: Help with understanding ip address problem

To answer your question: the number of bits reserved for subnetting is variable and the number of bits used depends on how many IP addresses you need to have in the subnet. When you established a requirement for 8 addresses, then the smallest subnet that has that many addresses uses 4 bits for subnetting. There is a solution that would work using 5 bits - but that would not be as efficient.

when you enter the new command it should over-write (or erase) the old command.

HTH

Rick

Community Member

## Re: Help with understanding ip address problem

Hi

I appreciate this may have not fit into the question, but just as a sanity check, could someone please verify my calculations.

If I want a subnet with a total of 8 ip's (so 6 usable ip's, one network, one broadcast)

Would the following sets of subnets be correct?

xxx.xxx.210.64-xxx.xxx.210.72

xxx.xxx.210.73-xxx.xxx.210.81

xxx.xxx.210.82-xxx.xxx.210.90

Dan

Silver

## Re: Help with understanding ip address problem

.72 is the network number for that subnet.

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