Understanding CIDR ranges (basic question!)

esuhl

One of our servers has IP authentication, logging users in automatically if they come from a recognised IP address. I'm normally asked to add IP ranges (such as 111.222.333.400 to 111.222.333.500), but this time I've been given a CIDR range.

I've used a CIDR calculator (http://bonomo.info/coyote/cidr-calculator.php) to convert the range from CIDR notation to a basic IP range that a simpleton like me can understand and for the (fake) CIDR range 147.147.147.147/28 I get an output like this:

Network = 147.147.147.144
Usable IPs = 147.147.147.145 to 147.147.147.158 for 14
Broadcast = 147.147.147.159
Netmask = 255.255.255.240

So... could a user be coming from the "Network" or "Broadcast" IP address? I presume not, but if not, what are these IPs used for? Some kind of local routing management?

Sorry if it's a dumb question - I'm not too hot with networking as you can probably tell!

John_Gray

Since you've got far too many 147s for my liking , you might like to translate this to the more common home network range of 192.168.1.0-192.168.0.255. This has a subnet mask of 255.255.255.0, the 'network' address is 192.168.1.0 and the broadcast address (by which all addresses can be contacted) is 192.168.1.255. Valid/usable user IP addresses in this range are 192.168.1.1 to 192.168.1.254 (ie, 254 addresses, being 256 minus the previous two).

Your data

Network = 147.147.147.144
Usable IPs = 147.147.147.145 to 147.147.147.158 for 14
Broadcast = 147.147.147.159
Netmask = 255.255.255.240

is just a bit more restricted. By the subnet mask 255.255.255.240 you have selected out 16 addresses of which the first is the 'network' address and the last the 'broadcast address'. User-usable IP addresses are the 14 in between, as it says.

You'll need to read up on the terms for a better explanation!

esuhl

Cheers John. My home network is all set up fine (since I didn't need to know about CIDR notation!) but this is part of a large corporate network - the 147s are just an example.

On my home network, my main PC is 192.168.0.2, the router is 192.168.0.1 on the local interface and whatever my ISP gives me on the Internet side.

I really don't know how corporate networks are set up, but from the above, I'm guessing that each of the usable IP addresses is assigned to a virtual router, and the device managing the routers gets the "network" address 147.147.147.144 and the "broadcast" address is used internally by the network to manage itself. So our servers will only see the "usable" IPs...? Is that even close to how it works?!

Anyway, thanks for your help - as you say, I probably just need to read up a bit to get my head round it!