Important concepts related to subnets

To Nha Notes | May 17, 2022, 6:52 p.m.

A network can be divided into subnetworks for easy maintenance and security concerns. These subnetworks are referred to as subnets. An IP address in CIDR format is divided into NID bits and host identifier (HID) bits. The netmask of the IP address represents the size of the NID bits. With subnetting, additional bits from the HID part will be used for subnetting. The netmask for a subnet will, therefore, represent the combination of NID bits and subnet identifier bits, and this combined bitmask is usually called a subnet mask.

We created a VPC with a CIDR block range of 10.0.0.0/16. Here, the netmask is /16 and represents NID bits. HID bits can be obtained by subtracting the netmask size from the total available 32 bits for an IPv4 address. So, there will be 16 HID bits, which means we can have up to (2^16) hosts, out of which five are reserved. We created subnets with a subnet mask of /24, so the HID bits for the subnet is 8. Therefore, we can have 2^8 IP addresses in each subnet, which is 256.

The first subnet has a block address of 10.0.1.0/24 and contains IP addresses from 10.0.1.0 to 10.0.1.255. The second subnet has an address of 10.0.2.0/24 and contains IP addresses from 10.0.2.0 to 10.0.2.255. In the Subnets page, however, we saw that there are only 251 available IP addresses. This is because five of the IP addresses are reserved by AWS.

We can also create a subnet with the same IP address range as that of the VPC. We selected Don't Assign Ipv6 for IPv6 CIDR block. Instead of doing this, we can provide a custom IPv6 address range here that is a subset of our VPC's IPv6 address range, or even the same as that of the VPC's IPv6 address range. We used the CIDR ranges in the subnet names. We can also add region details to the name. Since we selected No preference for Availability Zone, AWS will be selecting one for us.

Let's quickly go through some important concepts related to subnets in AWS:

• The first IP address of a subnet represents the subnet ID, while the last IP address represents the subnet's directed broadcast address. Therefore, we cannot use the first and last IP address of a subnet for hosts. AWS reserves additional IP addresses.
• The first IP address of the first subnet of a network represents the subnet ID, as well as the network's ID. Similarly, the last IP address of the last subnet of a network represents the subnet's and the network's directed broadcast address. When using these IP addresses from outside the network, they will be considered as the networks', and when using them within the network, they will be considered as the subnets'.
• A subnet in AWS VPC is always associated with one Availability Zone (AZ).
• While we cannot have one subnet associated with more than one AZ at a time, we can have multiple subnets associated with a single AZ.

Reference

https://heartin.tech/en/blog-entry/important-concepts-related-subnets