> For the complete documentation index, see [llms.txt](https://karansingh.gitbook.io/aws-saa-c02/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://karansingh.gitbook.io/aws-saa-c02/virtual-private-cloud-vpc/networking-for-vpcs.md).

# Networking for VPCs

## CIDR (**Classless Inter-Domain Routing)**

* The **formula to calculate the number of assignable IP address:**

  * Subtract the number of network bits from 32 and raise 2 to that power.
  * For example, a **/24 network has (2^(32-24)) is 256 addresses**.

* For example **for 10.0.0.0:**
  * A **/10 prefix** = **10.0.0.0 to 10.255.255.255**.
  * A **/16 prefix** = **10.0.0.0 to 10.0.255.255**.
  * A **/24 prefix** = **10.0.0.0 to 10.0.0.255**.\
    A **/32 prefix** = **10.0.0.0**.<br>

* **Bigger the IP network prefix** (the number after the slash) = **Smaller the range of IP addresses.**
  * For example, in AWS, your **VPC must be /16 or smaller**, which is anything in between /16 and /32.

## Private vs Public IP

* **Private IP address** of a system is the IP address which is used to **communicate within the same network**.
  * **`10.0.0.0 to 10.255.255.255 (Big networks, like offices)`**\
    **`172.16.0.0 to 172.31.255.255 (Default for AWS)`**\
    **`192.168.0.0 to 192.168.255.255 (Small networks, like homes)`**

* **Public IP address** of a system is the IP address which is used to **communicate outside the network**.
  * **Assigned by the ISP** (Internet Service Provider).