Cidr blocks AWS explanation

Classless Inter-Domain Routing (CIDR) blocks are for specifying a range to IP addresses in format of IPv4 or IPv6. For the sake of simplicity I will explain rest of this in format of IPv4 however it is applicable to IPv6.

General format for CIDR Blocks: x.y.z.t/p

x, y, z and t are numbers from 0 to 255. Basically, each represents an 8 bit binary number. That's why it is range is up to 255. Combination of this numbers becomes an IPv4 IP address that must be unique to be able to identify a specific instance.

In case of AWS, p is a number from 16 to 28. It represents the number of bits that are inherited from given IP address. For example: 10.0.0.0/16 represents an IP address in following format: 10.0.x.y where x and y are any number from 0 to 255. So, actually it represents a range of IP addresses, starting from 10.0.0.0 to 10.0.255.255.

However for each CIDR block, AWS prohibits 5 possible IP addresses. Those are the first 4 available addresses and the last available address. In this case:

1. 10.0.0.0: Network address
2. 10.0.0.1: Reserved for VPC router
3. 10.0.0.2: DNS server
4. 10.0.0.3: Reserved for future use
5. 10.0.255.255: Network broadcast

See here for official doc.

Actually this is one of the main reasons why AWS permits numeric value of p up to /28. Because for p=30, there will be 4 available values however AWS needs 5 IP address to use. In my opinion for p=29, they might found it inefficient to occupy 5 addresses to provide 3 possible IP address.

Number of possible IP addresses can be calculated by using this formula:

NumberOfPossibleIPs = 2^(32-p) - 5

Classless Inter-Domain Routing (CIDR) block basically is a method for allocating IP addresses and IP routing. When you create a network or route table, you need to specify what range are you working in. "0.0.0.0" means that it will match to any IP address. Some IP addresses are specific, like 10.0.0.0, which will match to any IP address beginning with 10. With any IP address range, you can be more specific by using a suffix(something like /32 from your example). These allow the notation to specify number of bits to be used from Prefix(actual IP-range like 10.0.0.0). It represents the bit length of the subnet mask, as indicated above. The subnet mask is like masking when painting. You place a mask over what you DO NOT want to paint on.

For example, 10.10.0.0/16 will have 256 * 256 IP address in its range.

NOTE: Some of the IP address in a range are reserved for various purposes. According to AWS VPC documentation, following are the reserved IP addresses.

• 10.0.0.0: Network address.
• 10.0.0.1: Reserved by AWS for the VPC router.
• 10.0.0.2: Reserved by AWS. The IP address of the DNS server is always the base of the VPC network range plus two; however, we also reserve the base of each subnet range plus two. For VPCs with multiple CIDR blocks, the IP address of the DNS server is located in the primary CIDR. For more information, see Amazon DNS Server.
• 10.0.0.3: Reserved by AWS for future use.
• 10.0.0.255: Network broadcast address. We do not support broadcast in a VPC, therefore we reserve this address.

Hope this helps!

All of the above answers are great, but are missing something pretty important for the people who don't understand addressing.

IP addresses are literally just a string of binary, broken up into 4 "octets". Each octet is a 2^8 block; 00000000. So to a machine, an IP address looks like this (with (.) added for human-ness):

00000000(.)00000000(.)00000000(.)00000000

When we're talking about the "mask" on the IP address, it means "the bits that don't change". The /8 or /255.0.0.0 on the end of the block signifies the number of bits that are not allowed to be used by this network.

So, lets say we have a CIDR block of 10.0.0.0/8 - this can also be written in the format 10.0.0.0/255.0.0.0, and you may in fact see this for of notation in older versions of linux. You will also note that 255 is the decimal representation of the binary string 11111111 - 8 binary "ones". So what the machine sees is the following:

 Net: 00001010(.)00000000(.)00000000(.)00000000
Mask: 11111111(.)00000000(.)00000000(.)00000000

The part of the mask with 0's is usable address space within the network.

So the following example addresses are valid in this network, because on the 0 parts of the masked range are changing:

00001010(.)00000001(.)00110000(.)00111000
00001010(.)00110001(.)00110100(.)00111001

When we say "cidr block" we simply mean "the human-readable shorthand way of expressing binary strings understood by a machine". In the above example, the first octet can be expressed as 10, and the latter octets 0. And the Mask can be expressed as 255 and the latter octets of 0, or; because the mask is always a sequence of 1's, then a sequence of 0's, the length of the 1's, i.e. 8

And as such, we get a cidr of 10.0.0.0/255.0.0.0, or 10.0.0.0/8

A few more examples:

-- 172.1.1.0/24
 net: 10101100.00000001.00000001.00000000
mask: 11111111.11111111.11111111.00000000
      ^ 24 bits for the mask     ^ 8 bits of usable space

-- 10.10.10.8/29
 net: 00001010.00001010.00001010.00001000
mask: 11111111.11111111.11111111.11111000
      ^ 29 bits for the mask.         ^ 3 bits of usable space

Importantly though, this is only one aspect of networking. Usually a couple of these are reserved for things. See other answers for AWS specific things. In their examples, the "first 4" ip addresses reserved for AWS will be the first 4 usable addresses, which would be

...00 - Network address
...01 - Router
...10 - DNS
...11 - Futureproofing