1.2.3. 💡 First Principle: Routing & Traffic Flow Control

Routing and traffic flow control fundamentally direct network packets between different network segments, ensuring proper connectivity, optimizing performance, and enforcing security policies.

Scenario: You need to ensure that traffic from your private application servers can reach the internet for software updates, but no inbound internet traffic should be allowed to them. Additionally, your web servers must be directly accessible from the internet.

Routing is the process of selecting a path across one or more networks. Traffic flow control involves managing how network traffic is directed and prioritized. These concepts are at the core of network operations and are critical for any AWS environment.

Key Concepts:

• IP Addressing (CIDR): (Classless Inter-Domain Routing) Used to define network address ranges for VPCs and subnets. Understanding CIDR is crucial for subnetting and preventing IP address conflicts.
• Route Tables: A set of rules (routes) that determine where network traffic from a subnet or gateway is sent. Each subnet in a VPC must be associated with a route table.
• Internet Gateway (IGW): Allows communication between a VPC and the public internet.
• NAT Gateway (Network Address Translation): Enables instances in private subnets to initiate outbound internet connections without being publicly exposed.
• VPC Peering: Connects two VPCs to allow communication between them as if they were on the same network.
• AWS Transit Gateway (TGW): A network transit hub that connects VPCs and on-premises networks, simplifying complex routing.
• Border Gateway Protocol (BGP): A routing protocol used with AWS Direct Connect and Site-to-Site VPNs for dynamic routing between your network and AWS.
• DNS (Domain Name System): Translates human-readable domain names into IP addresses, a critical component of traffic flow.

⚠️ Common Pitfall: Incorrectly configuring route tables, leading to black-holed traffic or unintended internet exposure. Always verify routes for both inbound and outbound traffic.

Key Trade-Offs:

• Public Accessibility vs. Security: Direct internet access via an IGW is simple but less secure. Using a NAT Gateway for outbound-only access from private subnets adds a layer of security and control but incurs cost.

Reflection Question: How do routing mechanisms (e.g., route tables, Internet Gateways, NAT Gateways) fundamentally direct network packets and control traffic flow between different network segments in a VPC, ensuring proper connectivity while enforcing security policies?

💡 Tip: Remember that every subnet must have a route table associated with it, even if it's the default one.