Something that came to mind

Alhaji265

Questions that came to my mind when studying for the CCENT:

• Why can't notations /31 and /32 can't be used for assigning IP addresses to host? I need some clarification.
• Is it possible to what you learn in studying for your CCNA?
• When subnetting how does the valid subnets comes to 256?

oli356

/31 = 1 host bit. Number of hosts: 2^1=2 then minus 2 (broadcast address and network) = 0. You can"t have 0 hosts.
/32, This would mean every bit out of the 32 is a network bit. You can't have this obviously.

You NEED to know the formula to work out:
Number of hosts per subnet: 2^x-2 when x is the number of host bits (0s)
Number of subnets: 2^x when x is the number of network bits (1s)

Don't quite understand what you mean about the 256. I might have answered it above with the formula.

dead_p00l

I think oli356 explained it pretty well. As far as the 256 you start with 0 and go to 255 giving you 256. Outside the scope of the ccna /31 and /32 addresses can be used. /31's typically for point to point links and /32's typically for loopback addresses. Hopfully this doesnt cause more confusion.

Somnipotent

Actually a /31 CAN be used, however it's not common place. Refer to RFC 3021 for /31 usage between IPv4 point-to-point networks (RFC 3021 - Using 31-Bit Prefixes on IPv4 Point-to-Point Links). I deal with quite a few customers who use it.

/32 is an all one's address, a broadcast address space meaning ALL hosts in this network, or as stated, a HOST address specifically that particular host as in loopback interfaces.

beat me to the punch... blah

oli356

Learn something new everyday! I don't get that /31 and how it can be used for point to point though, I don't care for now Hopefully I will get to study for my CCNP and find out then