Let’s get a quick overview of Routing and Route Tables in AWS.
You will learn
- What is routing?
- What is route table?
- How does routing happen on internet?
- How does routing happen in AWS?
- What is VPC Route Table?
- What is Subnet Route Table?
Routing on the internet
- You have an IP address of a website you want to visit
- There is no direct connection from your computer to the website
- Internet is actually a set of routers routing traffic
- Each router has a set of rules that help it decide the path to the destination IP address
Routing inside AWS
- In AWS, route tables are used for routing
- Route tables can be associated with VPCs and subnets
- Each route table consists of a set of rules called routes
- Each route or routing rule has a destination and target
- What CIDR blocks (range of addresses) should be routed to which target resource?
- Rule 1 - Route requests to VPC CIDR 172.31.0.0/16 (172.31.0.0 to 172.31.255.255) to local resources within the VPC
- Rule 2 - Route all other IP addresses (0.0.0.0/0) to internet (internet gateway)
Execution of Route Table
- What happens if I search for an address 172.31.0.10? - Two destinations match - 172.31.0.0/16 (172.31.0.0 to 172.31.255.255) and 0.0.0.0/0 - The most specific rule wins. 172.31.0.0/16 is more specific - Result : Routing to a local resource
- What happens if I search for an address 188.8.131.52? - One destination match - 184.108.40.206 - Result : Routing to internet
- The most specific matching route wins
VPC Main Route Table
- Each VPC has a main route table, by default
- Main route table has a default route enabling communication between resources in all subnets in a VPC
- Default route rule CANNOT be deleted/edited
- HOWEVER you can add/edit/delete other routing rules to the main route table
Subnet Route Tables
- Each subnet can have its own route table OR share its route table with the VPC
- If a subnet does not have a route table associated with it, it implicitly uses the route table of its VPC
- Multiple subnets can share a route table
- HOWEVER at any point in time, a subnet can be associated with one route table ONLY
VPC Route Table and Routing - AWS Certification Exam Practice Questions