How to interpret an IP address block?

Question

Hi all,

If I have a block of private IP addresses such as 171.58.0.0/12, does this mean that I essentially bitwise AND the 32-bit version of 171.58.0.0 with 32 bits of 1's, the last 12 of which are 0'd out, to get the longest prefix of acceptable private IP addresses in that range?

10101011.00111010.00000000.00000000 (171.58.0.0) AND
11111111.11111111.11110000.00000000 (12 bit mask?)

=

10101011.00111010.00000000.00000000 (Longest Prefix of Private IP addresses)

to get the prefix acceptable private IP addresses?

Thanks!

Answer 1

You make a 32 bit number which has the higher 12 bits set to one, then AND it to the given IP address, and you get the network address. The remaining bits are available for host addresses, except the reserved numbers of all zeros (the network address itself) and all ones (the broadcast address). In your case it would be:

171.58.0.0 = network
171.58.0.1 to 171.58.15.254 = hosts
171.58.15.255 = broadcast

EDIT. See Tony van der Peet's answer since it adds valuable information to my answer.

Answer 2

In a word, yes.

http://en.wikipedia.org/wiki/IPv4%5Fsubnetting%5Freference

http://en.wikipedia.org/wiki/Classless%5FInter-Domain%5FRouting

http://www.faqs.org/rfcs/rfc1519.html

Answer 3

@Konamiman - your answer is pretty much spot on (+1), but I couldn't avoid answering because of the strange stuff in the question.

First of all, I would like to highlight that the OP got the 12 bits inverted, /12 means 12 bits of 1's one the left, not 12 bits of zeroes on the right.

Second, 171.58.0.0/12 in binary is 10101011.00111010.00000000.00000000 (as the OP states) with a mask of 11111111.11110000.00000000.00000000, which means that you have non zero bits in the host address part of the address. As far as I know, this is not the way to express a block of addresses, even though it is a valid single address in that subnet.

More sensibly, you would express this as 171.48.0.0/12 (is this a typo in the question?), which means that the host part is all zeroes. The smallest valid netmask for 171.58.0.0 would be /15.

EDIT: Take a look at this IP addressing guide. It's a bit out of date since it refers to class A, B, C, but is a useful cheat sheet for netmasks.

Answer 4

Assuming that this is based upon CIDR, maybe`

Mask:255.240.0.0     Host/Net - 1048574
Network          Broadcast
171.48.0.0       171.63.255.255