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CIDR Blocks
The CIDR block (Classless Inter-Domain Routing) we chose when we created the VPC tells us how many addresses can exist inside the VPC.
CIDR is pronounced "sih-der."
No, it's pronounced "sigh-der." I'm the author, Lane, go back to YouTube.
This isn't a networking course, but we'll cover the basics of IP addressing.
IPv4 addresses (the "normal" kind) have four segments: A.B.C.D.
Each segment is a number from 0 to 255 (an 8-bit unsigned integer). After a segment hits 255, it rolls over to 0, and the previous segment increments. For example:
10.20.30.255 + 1 = 10.20.31.0
10.20.255.255 + 1 = 10.21.0.0
10.255.255.255 + 1 = 11.0.0.0
An IPv4 address is a 32-bit number split into four 8-bit segments. The lowest possible address is 0.0.0.0, and the highest is 255.255.255.255 (though much of that space is reserved for special purposes).
CIDR describes a block of IP addresses in terms of a starting address and a size:
starting.address.goes.here/size
The size suffix follows the formula 2^(32 - size) = number_of_addresses. So a larger suffix actually means fewer addresses. Here are some examples:
| CIDR | Size suffix | Number of addresses |
|---|---|---|
| 10.25.37.123/32 | 32 | 1 |
| 10.25.37.0/24 | 24 | 256 |
| 10.25.0.0/16 | 16 | 65,536 |
| 10.0.0.0/8 | 8 | 16,777,216 |
You may have heard about another kind of addressing called IPv6, and may have encountered its advocates in the wild. It's a newer standard for IP addressing that supports 340 undecillion addresses, rather than the ~4.3 billion addresses in IPv4, but its support by many cloud providers is still... lackluster.