Solving an argument on "255" in IP's

[mikelovellgb]

Someone made the following claim, the IP's:

10.255.255.81

192.168.255.81

Are invalid because "octets of 255 are not valid in unicase IP addresses."

I say that's not true, that "255" can be used anywhere, and can in some circumstances

even be used in the last octet.  It doesn't automatically invalidate a unicase/host IP.

He cites:

http://www.comptechdoc.org/independent/networking/guide/netaddressing.html

"255 - The value of 255 is never used as an address for any part of the IP address. It is reserved for broadcast addressing. Please remember, this is exclusive of CIDR. When using CIDR, all bits of the address can never be all ones."

Who is correct, and why?

Thanks,

[cadet alain · Accepted solution]

Hi,

you are correct, what is not a valid host address is one with all host bits set to 1 and to know which bits are host bits you

have to take the prefix length or netmask into account.

Regards.

Alain

Don't forget to rate helpful posts.

[Karsten Iwen · Accepted solution]

That's not the only wrong statement in that guide ... So better ignore that completely.

But that misbelief was quite common some time ago. I once had a NAS where I also couldn't configure a "255" in the second or third octet, which was bad as my home-network was 10.255.255.0/24 ...

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