Netmasks for dummies 3008

StupidScript

Well, I said I was cheating - that is an example of a CIDR buttignment. It often is not possible to code in one mask, and you have to use more than one mask to cover the lot.

That was the correct idea - it's two 256s and a 208, which can be coded as a 512, a 128, a 64 and a 16, or

255.255.254.0 255.255.255.128 255.255.255.192 255.255.255.240

OK, you got an '8' to return 255.0.0.0, and '29' as 255.255.255.248. Let's write these numbers out in binary... stop groaning, it's not that bad. There are nine valid numbers used for each octet of the mask. These are 255, 254, 252, 248, 240, 224, 192, 128, and 0. In binary, these are:

255 1111 1111 248 1111 1000 192 1100 0000 254 1111 1110 240 1111 0000 128 1000 0000 252 1111 1100 224 1110 0000 0 0000 0000

If you're not familiar with this scheme, the right most digit has a value of one, and each digit to the left doubles the value. This means 128 64 32 16 8 4 2 1. A "128" plus "64" gives 192. and so on.

Now, here are those two masks you mention

255.0.0.0 1111 1111 0000 0000 0000 0000 0000 0000 255.255.255.248 1111 1111 1111 1111 1111 1111 1111 1000

And the question you have to answer is "how many 'ones' (1) are there in each mask. Looks to me like 8 in the first, and 29 in the second. ;-)

This is the number of 'bits' that form the network portion of the mask. (The zeros are the host portion of the mask.)

OK - 8 x 256 (196.25.32, 196.25.33, 196.25.34, 196.25.35, 196.25.36, 196.25.37, 196.25.38 and 196.25.39). The first answer is what you'd see in the output ofsbin-ifconfig orsbin-route, while the third one is what is commonly called a CIDR value, because there are 21 "ones" in the mask. All well and good. The middle answer is quite confusing, but is simply the inversion of the first mask - I suppose you could call it the 'hostmask' (as opposed to 'netmask'). The author of the web page doesn't explain why he's providing that mask, as it's not a value that is used in any command.

Actually, if you plug in the 194.9.124.0 - 194.9.126.207 values into the web page, it does return the four binary masks I show above. In my original response, I mentioned that the block owner probably has broken this block up behind his gateway. For routing to this address (what it's really used for), your computer really doesn't care. It looks at it's own routing table, doesn't see a matching answer, and therefore pbuttes it to the 'default gateway' ("I don't know where this packet goes - your handle it for me, OK?") Maybe your default has to pbutt it on to it's default, but eventually, some router up the line is going to know to send the packets to mumble.foo.bar.baz (no, I don't know it's address) but that's the gateway router to 194.9.124.0-23 (and to 194.9.126.0-25, and...) Beyond that router, there might be one, or a hundred networks - the block owner hasn't said.

Well, it does make a difference, because that is actually a 'hostmask', and it's not common. There are no network tools that I'm aware of that know anything about it, and I suspect few people. The only reason I caught it immediately was seeing it along side the other masks.

Are we having fun yet? ;-)

Old guy