Another, more reasonable interpretation:
what does 192.168.0.1/24 mean? {or, equivalently,
192.168.0.1/255.255.255.0) ...
what does 192.168.0.0/24 mean?
It means, "My Address is 192.168.0.1," and "I belong to the subnet with addresses that range from 192.168.0.0 through 192.168.0.255"
The rules: 1. The dotted-quad is a specific address (save for special interpretations of the first and last addresses in the range). It must be unique on the network (i.e., nobody else using it).
2. The mask 255.255.255.0 (or, in CIDR, /24) specifies what *other* addresses (the subnet range) this NIC can communicate with (including itself)
See if this made-up example makes sense:
My NIC's address: 75.213.18.72 My "neighborhood" of other addresses on this subnet: 75.213.18.0 through 75.213.19.255 There are 512 addresses in the range; 512 is equivalent to a 9-bit binary value. To compute CIDR: 32 - (9) = 23 So, my NIC's address and mask are: 75.213.18.72/23 or, if you prefer, 75.213.18.72/255.255.254.0
Here's a completely different way to look at how to understand IP addresses from one computer on a network (I'm not teaching routing, here :-) For a 512 address range, it takes 9 bits to encode those 512 addresses. Now, look at the 32-bit address: The least-significant 9 bits identify addresses WITHIN the subnet; the rest of the bits are identical for everything WITHIN this subnet, and something else for NICs OUTSIDE the network.
I think of the MASK as identifying, in a concise way, the line between the bits to the left, and bits on the right. If a packet comes to me and the bits to the left of that line don't match mine, I ignore the packet. If that *does* match, then I see if the message has my unique bit pattern; if not, then I ignore it. If it passes both tests...It's For Me!)
Oh, and your second question:
what does 192.168.0.0/24 mean?
It means: The entire subnet from 192.168.0.0 through 192.168.0.255.
Remember, the lowest subnet range address (all bits = 0), is the address of the entire network, and the highest subnet range address (all bits = 1), is the broadcast address (to which all active addresses in the subnet range listen)
Try working it out yourself, to see if you "get" these IP addressing rules.
--Carol Anne
-----Original Message----- From: centos-bounces@centos.org [mailto:centos-bounces@centos.org] On Behalf Of mouss Sent: Thursday, February 07, 2008 1:21 PM To: CentOS mailing list Subject: Re: [CentOS] Security help desperately needed - more info
Milton Calnek wrote:
Michael Simpson wrote:
Hi there,
Should the IP address supplied be the actual address for
eth0 rather
than the network address?
ie 192.168.0.1/24 rather than 192.168.0.0/24
I dunno... what does 192.168.0.1/24 mean?
this one is not always accepted.
what does 192.168.0.0/24 mean?
this is the correct one.
The way I see it, they both mean 192.168.0.0 - 192.168.0.255.
yes, but the first version is not accepted by all software. because no IP will satisfy bin(ip) & 0xffffff00 = bin(192.168.0.1)
anyway, I have a samba setup with interfaces = 192.168.10.0/24 and it works. so this is not the source of the problem.
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