CIDR Range

bornwith

I am coming confused about the possible range of CIDR masks. I would assume any value between /9 and /27 would be valid but I have seen some sources say /13 and /27. What is the correct answer.

Thanks.

dontstop

Could you elaborate a bit more? I'm not sure I follow the question. I thought CIDR meant you could theoretically use any number of masking bits for the address as class rules go out the window (unless you're already dealing with an allocated fixed block)

PCTechLinc

A CIDR notation is just a simpler way to show what subnet mask has been administratively assigned, by representing the numbers of bits being allocated for the network portion of the address.

Instead of putting:
192.168.1.0 / 255.255.255.0

CIDR = 192.168.1.0/24

What is "correct" depends on how many networks / hosts per network you want, based on the Classful boundary of the original network. Practically speaking, the range is from /8 through /31, as long as you're not "supernetting" or "summarizing".

A quick Google search showed me this page, which explains CIDR in some detail: CIDR Notation Explained Simply | Tom Pimienta – Software Developer

winslade

For your subnet mask it consists of 32 1's or 0's
the CIDR is simply reflecting how many 1's there are

For example, /13 would equal 11111111.11111000.00000000.00000000
Convert binary to decimal and your mask equals 255.248.0.0

/1 would equal 10000000.00000000.00000000.000000000, which is 128.0.0.0 in decimal
/32 would equal 11111111.11111111.11111111.11111111, which is 255.255.255.255 in decimal

rob42

winslade wrote: »

For your subnet mask it consists of 32 1's or 0's
the CIDR is simply reflecting how many 1's there are

For example, /13 would equal 11111111.11111000.00000000.00000000
Convert binary to decimal and your mask equals 255.248.0.0

/1 would equal 10000000.00000000.00000000.000000000, which is 128.0.0.0 in decimal
/32 would equal 11111111.11111111.11111111.11111111, which is 255.255.255.255 in decimal

Except...

The minimum is /2 and maximum is /30, but even that depends on the IP Address Class...

With Class C, the minimum is /3

dontstop

I've read that you can use /31's for point to point links.

http://packetlife.net/blog/2008/jun/18/using-31-bit-subnets-on-point-point-links/

rob42

dontstop wrote: »

I've read that you can use /31's for point to point links.

Using /31 subnets on point-to-point links - PacketLife.net

A 'corner case', but yes, technically, as you don't need a broadcast.

(a very good site, Packetlife I love the '**** Sheets')

bornwith

That was an interesting read. I didn't think that was possible.