| Linux ipnatctl HOWTO: Selecting What Packets To Mangle | If youre not familiar with the /-notation used for network addresses, it works like this.
Every machine needs an individual address. To keep things simple, we assign them in clumps; each network of machines generally gets a range of addresses.
A single IP address is 32 bits long: printed in binary has 32 binary
digits, each 1 or 0. Its standard to print them as 4 decimal numbers,
each representing 8 bits, such as 192.168.1.1. In binary, this would be 11000000101010000000000100000001.
To talk about a network of machines, we simply decide which bits of the address indicate the network: if two IP addresses have these bits the same, then they are on the same network.
The simplest way of representing this is the /n notation, such as 192.168.1.0/24. The /24 means that the first 24 bits of this address are important: remembering that each number represents 8 bits, this means that any IP address starting with 192.168.1. is on this network.
As another example, 10.0.0.0/8 means that any IP address starting with 10. is on the network.
The second part usually called the netmask doesnt have to be a multiple of eight, but it often is, to make it easier to instantly recognise related IP addresses.
There is another way to write the netmask, and that is to spell it out
in IP-address form, such as 192.168.1.0/255.255.255.0. In binary, 255 is 11111111, so 255.255.255.0 is 11111111111111111111111100000000. Each 1 in this netmask indicates that the corresponding part in the IP address is important: this means that the first 24 bits are important, so 192.168.1.0/255.255.255.0 is equivalent to ``192.168.1.0/24.
Here is a table of common addresses you will see:
Decimal Range Use
| 192.168.1.0/24 192.168.1.0-192.168.1.255 Common in masqueraded networks | 192.168.0.0/16 192.168.0.0-192.168.255.255 RFC 1918: private network addrs | 10.0.0.0/8 10.0.0.0-10.255.255.255 RFC 1918: private network addrs | 172.16.0.0/12 172.16.0.0-172.31.255.255 RFC 1918: private network addrs
This can be quite confusing, but youll see the same notation and the same numbers popping up over and over again, so I recommend you read the above paragraphs a couple of times.