## CIDR

See also RoutingUp till now, I've been talking about IP addresses and specifying the netmasks by spelling them out: network 192.168.13.0 with a netmask of 255.255.255.0. I've explained that the netmask indicates the bits that are the network part of the address, and that changing anything in those bits puts you on a different network or subnet.

Is there any real difference between a network and a subnet? Not really. Any network is a subnet of something larger, so in that sense, the terms are identical. However, you could look at this another way: your network is the addresses which you can subnet. Or, your network is the bits you cannot change because someone else assigned them to you. As it's always just the number of bits that is important, we can represent networks or subnets another way. The network 192.168.0 with that 255.255.255.0 netmask can be expressed as 192.168.13.0/24. The "24" is the number of bits set to "1" (remember, 8 bits in each section of a mask).

So, a 255.0.0.0 netmask would be /8, a 255.255.0.0 would be /16
and so on. Those are pretty easy. What about masks like
255.255.240.0? If you aren't used to thinking in bits, this might
give you a little headache. But don't panic, it's not that hard.
One way to think of it is how many bits are **not** set in the
third octet. We have 8 each set in the first two, so that's 16, and
it would be 24 if all the bits were set in the third, but bits
adding up to 15 (255 - 240) are missing. That's the 8-4-2-1 bits (8
+ 4 + 2 + 1 = 15), so 4 bits are missing, so it is a 20 bit mask:
/20.