IP address wrong?

[AlexLi]

Recently I detached a laserjet 4v printer from a computer and made the printer as a network node and assinged IP address 172.26.19.0 to it. I has been working well for a few weeks now. Today someone saw the IP address and told me it was an illegal IP address. He said I can not use 0 or 255 in the last octet. He did not find any conflict or any impact by my "wrong" IP address on the network or anything like that. He just happend to see that address and thought it was wrong. I feel he is right. I remmebr reading somewhere about that says 0 and 255 should not be used. But can anybody explaninto me why it has been working fine so far. Or can any one tell me if one used 0 or 255 by mistake what will happen on the network? Thank you.

[Jerome Henry · Accepted solution]

Hi,

Basically it depends on your netmask (to say simply : it's not that easy). We have to turn to binary stuff to make it clear. Each number in 172.26.19.0 is in fact a byte, 8 bits, which value can be from 00000000 to 11111111. When we write 0, it's in fact 00000000, and 255 is 11111111. Converting froms bits to decimal numbers is not really easy the first time. Here is a guide :
http://www.ib-computing.com/Concluding24.htm

Why do we need all that ? Because the problem in your question is to know what is the mask. To pu it simply, if your address is 172.26.19.1, for example, and your mask 255.255.255.0, this means (this mask) that the first three numbers are common to all the computers in your network. And the last (which is on 0 in the mask) is specific to each computer.
So 172.26.19.1 and 172.26.19.251 are possible values on the same network. 172.26.19.257 doesn't exist, as 11111111 (max possible value) is 255. And 172.26.18.25 is on anotehr network, as 18 is not a changeable value for this network.
Now, i this scenario, there are 2 specific numbers : 172.26.19.0 and 172.26.19.255. The last is called broadcast, you will call this address when you want all the computers in your network to fill concerned. So you can't use this address for a single machine.
172.26.19.0 is called the network : it's a way of commonly designate the network itself, a kind of 'mask like', where the first numbers are your network, and the part that changes is on 0.
So, normally, this number is forbidden. Nevertheless, many equipments (as Windows family) are network tolerant, as they can address such a forbidden address. The idea is that if you have the mask (255.255.255.0), then you know the network, and this 'network address' is redundant.
But as far as the protocol is concerned, this address should be forbidden. It won't work in many systems, depending on your routers and servers brand, they just won't recognize your address, and ignore your printer (you can't print)...

But there is a last issue, less easy to get (don't read it if you are afraid of being mixed).
The system described above, 172.26.19.0, 255.255.255.0 means that the part that changes is the last digit, so the values can be from 0 to 255, 0 and 255 themselves being forbidden. So you can have 254 machines on your network (0 to 255 is 256 values, 2 are forbidden). But what happens if you need 450 addresses ? The we have to do it another way, and make a network in which the changeable part is bigger... For that, we need to turn to binary. One valid address is 172.26.19.1 ? That's 10101100.00011010.00010011.00000001.
IN the former scenario, 10101100.00011010.00010011 are fixed, and last part changes. If we need a bigger network, then we'll have to make the last binary digit of the last fixed item to change. See :
10101100.00011010.00010011.00000001, we'll choose to make 1.00000001 change, and 10101100.00011010.0001001 to be common. It's a good idea, because in having 9 digits change instead of 8, then we can have 512 solutions, and we can address our 450 machines easily. But our mask will change ! The possible values on our new network wil be from 10101100.00011010.0001001 0.00000000 to 10101100.00011010.0001001 1.11111111. Let's convert back to decimals :
10101100.00011010 stay 172.26, 0001001 0.00000000 is 18.0 and 0001001 1.11111111 is 19.255. So the network adress will be 172.26.18.0 (this address is forbidden), and the broadcast address 172.26.19.255 (this addres is forbiden too). What about the mask ? To calculate it, we turn the values that remain common to 1, and the values that change to 0. In binary, the mask will be :
11111111.11111111.1111111 0.00000000. Turning it to decimal, the mask is 255.255.254.0 (so you understand now why the mask was 255.255.255.0 before : 11111111.11111111.11111111.0).
Why all that ? Because in this case, 172.26.19.0 is not the network address, it's just one address among others...

So to answer your question, it's true if your mask is 255.255.255.0, but not necessary true if your mask is different. Got it ?

J

[AlexLi]

Jerome, that's brilliant... Our subnet mask is 255.255.252.0. Thank you so much.

[Jerome Henry]

Glad to help !

:]]

J