Subnetting/IP Address Question...

[eggedd2k]

Can anyone tell me the best method for working out the 'usable host addresses' from a given network address?

also on questions which ask you to specify the least usable host address from a given network/subnet address.

thanks

 

[Lundah]

(2^n) - 2 is what I use, with "n" being the number of host bits. Easy way to get the number of host bits? Just take 32 - x, where "x" is the number of mask bits.

 

[ReadRight]

Figure out the range first.

i.e. if subnet mask is 255.255.255.240
you know the range is 16, because 256-240 = 16

armed with that knowledge, apply it the network:

if you already know the network, i.e. 192.168.1.32, it's easy. The range is 16, so the next network above yours is 192.168.1.48. I just added 16 to 192.168.1.32 to figure that out. Counting down you skip .47 cause it's the broadcast, so your last usable address is .46

So,

192.168.1.32 /28 --- your network

192.168.1.33 - first usable address
...
...
192.168.1.46 - last usable address

192.168.1.47 - broadcast address
192.168.1.48 - next network address