Quick IP address question

Jimbo Mahoney · 08-16-2001, 04:58 PM

What does the /24 and /25 etc mean on the end of an address?

e.g. 192.168.1.0/24 - I believe this means all addresses from .0 to .255?

how is it worked out?

I have searched for this info with google for a while now to no avail!

I don't understand it!

192.168.1.0/25 means .0 to .128! (I think)

How does that work?

Regards,

Jimbo

jharris · 08-16-2001, 05:46 PM

Quote:

Originally posted by Jimbo Mahoney
What does the /24 and /25 etc mean on the end of an address?

e.g. 192.168.1.0/24 - I believe this means all addresses from .0 to .255?

how is it worked out?

Yeah thats right. The /24 is the number of bits that are masked off the IP address to work out your local subnet. For example your IP address is represent as a 32 bit binary number, so your above example of 192.168.1.0 is

Code:

00000011 00010101 10000000 00000000

and your netmask is

Code:

11111111 11111111 11111111 00000000

so when you mask off the IP address using your netmask (every bit set on the netmask masks out the corresponding bit on your IP address leaving you the maximum number of machines in your subnet) so

Code:

00000011 00010101 10000000 00000000
11111111 11111111 11111111 00000000
leaves you with 8 bits =>  ********

giving you a max number of 255 - but 255 is reserved so you can only use 0 - 254.

Quote:

Originally posted by Jimbo Mahoney
I don't understand it!

Make more sense now? I haven't tried to explain that in ages so excuse me if its a little rough. Get yourself a good book on TCP/IP if you want to know more - O'Reilly offer a good one http://www.oreilly.com/catalog/coreprot if you are interested.

cheers

Jamie...

Jimbo Mahoney · 08-16-2001, 05:50 PM

good work fella!

I undestand it a bit better now.

Thought it might be a binary thing.

Regards,

Jimbo