# binary and IP addresses

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Hi

I was wondering if someone could clear something up for me:

when converting decimal to binary then 255 = 11111111 or 8 places:  128 64
32 16 8 2 4 1

When dealing with subnet masks and hosts, to get 512 hosts you have to use 9
places ie

256 128 64 32 16 8 2 4 1 but all ones add up to 511?   what am I doing wrong
here?

Re: binary and IP addresses

You are forgetiing to add the .0 address on the next subnet.

It's easier to use the whole octet (ie 255 not 256) and add 2 (1 per
octet). 255 128 64 32 16 8 4 2 1 + 2  = 512

That does NOT give you 512 hosts anyways, it gives you 510 hosts, 1
network and 1 broadcast.

You do know about 2^N formula correct?

2^N = number of addresses including network and broadcast.
2^N-2= number of hosts

2^9-2=510

I found writing out the table a couple dozen times commits it to memory
quite easily.

Re: binary and IP addresses
Hi

255 is the last number from 0 ( zero )

form INCLUDING zero  to 255 is 256

00000000    =    0
11111111    =    255
-----------------------------

dont make the mistake with hosts
A subnet can handle x hoste -2 ( network and host address !!

192.168.0.0 / 24 also 255.255.255.0

192.168.0.1    Fist host
192.168.0.254    Last host

a subnet of 8 free bits can be seen as 256 -2  == 254 host !

Rijk van Harn
Ex CCNA ( planning exam next month )

Re: binary and IP addresses
Thank you all!  guess I need to revisit subnetting :)

Regards

Mark