# VPC
- [VPC](#vpc)
- [VPC \& Subnets Primer](#vpc--subnets-primer)
- [Internet Gateway (IGW)](#internet-gateway-igw)
- [NAT Gateway](#nat-gateway)
- [NAT Gateway vs NAT Instance](#nat-gateway-vs-nat-instance)
- [Network ACL \& Security Groups](#network-acl--security-groups)
- [Network ACLs vs Security Groups](#network-acls-vs-security-groups)
- [VPC Flow Logs](#vpc-flow-logs)
- [VPC Peering](#vpc-peering)
- [VPC Endpoints](#vpc-endpoints)
- [Site to Site VPN \& Direct Connect](#site-to-site-vpn--direct-connect)
- [Transit Gateway](#transit-gateway)
- [VPC Summary](#vpc-summary)
## VPC & Subnets Primer
- VPC -Virtual Private Cloud: private network to deploy your resources (regional resource)
- Subnets allow you to partition your network inside your VPC (Availability Zone resource)
- A public subnet is a subnet that is accessible from the internet
- A private subnet is a subnet that is not accessible from the internet
- To define access to the internet and between subnets, we use Route Tables.
## Internet Gateway (IGW)
- Connects a VPC to the internet.
- Allows instances in the VPC to directly communicate with the internet.
- Essential for a public subnet in a VPC to send/receive traffic to/from the internet.
## NAT Gateway
- Allows instances in a private subnet to initiate outbound traffic to the internet.
- Prevents unsolicited inbound traffic from reaching those instances.
- Used for scenarios where instances need to download patches, updates, etc., but should not be directly accessed from the internet.
- Managed by AWS
## NAT Gateway vs NAT Instance
| Attribute | NAT gateway | NAT instance |
| -------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
| Availability | Highly available. NAT gateways in each Availability Zone are implemented with redundancy. Create a NAT gateway in each Availability Zone to ensure zone-independent architecture. | Use a script to manage failover between instances. |
| Bandwidth | Scale up to 100 Gbps. | Depends on the bandwidth of the instance type. |
| Maintenance | Managed by AWS. You do not need to perform any maintenance. | Managed by you, for example, by installing software updates or operating system patches on the instance. |
| Performance | Software is optimized for handling NAT traffic. | A generic AMI that's configured to perform NAT. |
| Cost | Charged depending on the number of NAT gateways you use, duration of usage, and amount of data that you send through the NAT gateways. | Charged depending on the number of NAT instances that you use, duration of usage, and instance type and size. |
| Type and size | Uniform offering; you don’t need to decide on the type or size. | Choose a suitable instance type and size, according to your predicted workload. |
| Public IP addresses | Choose the Elastic IP address to associate with a public NAT gateway at creation. | Use an Elastic IP address or a public IP address with a NAT instance. You can change the public IP address at any time by associating a new Elastic IP address with the instance. |
| Private IP addresses | Automatically selected from the subnet's IP address range when you create the gateway. | Assign a specific private IP address from the subnet's IP address range when you launch the instance. |
| Security groups | You cannot associate security groups with NAT gateways. You can associate them with the resources behind the NAT gateway to control inbound and outbound traffic. | Associate with your NAT instance and the resources behind your NAT instance to control inbound and outbound traffic. |
| Network ACLs | Use a network ACL to control the traffic to and from the subnet in which your NAT gateway resides. | Use a network ACL to control the traffic to and from the subnet in which your NAT instance resides. |
| Flow logs | Use flow logs to capture the traffic. | Use flow logs to capture the traffic. |
| Port forwarding | Not supported. | Manually customize the configuration to support port forwarding. |
| Bastion servers | Not supported. | Use as a bastion server. |
| Traffic metrics | View CloudWatch metrics for the NAT gateway. | View CloudWatch metrics for the instance. |
| Timeout behavior | When a connection times out, a NAT gateway returns an RST packet to any resources behind the NAT gateway that attempt to continue the connection (it does not send a FIN packet). | When a connection times out, a NAT instance sends a FIN packet to resources behind the NAT instance to close the connection. |
| IP fragmentation | Supports forwarding of IP fragmented packets for the UDP protocol.
Does not support fragmentation for the TCP and ICMP protocols. Fragmented packets for these protocols will get dropped.
| Supports reassembly of IP fragmented packets for the UDP, TCP, and ICMP protocols. |
## Network ACL & Security Groups
- NACL (Network ACL)
- A firewall which controls traffic from and to subnet
- Can have ALLOW and DENY rules
- Are attached at the Subnet level
- Rules only include IP addresses
- Security Groups
- A firewall that controls traffic to and from an ENI / an EC2 Instance
- Can have only ALLOW rules
- Rules include IP addresses and other security groups
### Network ACLs vs Security Groups
| Security Group | Network ACL |
| ------------------------------------------------------------------------------------------------------------------------------------------------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------- |
| Operates at the instance level | Operates at the subnet level |
| Supports allow rules only | Supports allow rules and deny rules |
| Is stateful: Return traffic is automatically allowed, regardless of any rules | Is stateless: Return traffic must be explicitly allowed by rules |
| We evaluate all rules before deciding whether to allow traffic | We process rules in number order when deciding whether to allow traffic |
| Applies to an instance only if someone specifies the security group when launching the instance, or associates the security group with the instance later on | Automatically applies to all instances in the subnets it's associated with (therefore, you don't have to rely on users to specify the security group) |
## VPC Flow Logs
- Capture information about IP traffic going into your interfaces:
- VPC Flow Logs
- Subnet Flow Logs
- Elastic Network Interface Flow Logs
- Helps to monitor & troubleshoot connectivity issues. Example:
- Subnets to internet
- Subnets to subnets
- Internet to subnets
- Captures network information from AWS managed interfaces too: Elastic Load Balancers, ElastiCache, RDS, Aurora, etc…
- VPC Flow logs data can go to S3 / CloudWatch Logs
## VPC Peering
- Allows direct network connectivity between two VPCs.
- VPCs can be in the same AWS account or different accounts.
- Ensures private, high-speed communication between VPCs without routing traffic through the internet.
- Make them behave as if they were in the same network
- Must not have overlapping CIDR (IP address range)
## VPC Endpoints
- Endpoints allow you to connect to AWS Services using a private network instead of the public www network
- This gives you enhanced security and lower latency to access AWS services
- VPC Endpoint Gateway: S3 & DynamoDB
- VPC Endpoint Interface: the rest
## Site to Site VPN & Direct Connect
- Site to Site VPN
- Connect an on-premises VPN to AWS
- The connection is automatically encrypted
- Goes over the public internet
- On-premises: must use a Customer Gateway (CGW)
- AWS: must use a Virtual Private Gateway (VGW)
- Direct Connect (DX)
- Establish a physical connection between on-premises and AWS
- The connection is private, secure and fast
- Goes over a private network
- Takes at least a month to establish
## Transit Gateway
- AWS Transit Gateway allows you to connect different Amazon VPCs and on-premise networks through a centralized routing service.
- It can connect thousands of VPCs and on-premises networks through a single gateway, thus it has advanced security and routing features such as monitoring traffic on your network connections and defining routing policies.
- Provides a simple and scalable solution for managing large numbers of VPCs and network connections in your organization.
## VPC Summary
- VPC: Virtual Private Cloud
- Subnets:Tied to an AZ, network partition of the VPC
- Internet Gateway: at the VPC level, provide Internet Access
- NAT Gateway / Instances: give internet access to private subnets
- NACL: Stateless, subnet rules for inbound and outbound
- Security Groups: Stateful, operate at the EC2 instance level or ENI
- VPC Peering: Connect two VPC with non overlapping IP ranges, nontransitive
- VPC Endpoints: Provide private access to AWS Services within VPC
- VPC Flow Logs: network traffic logs
- Site to Site VPN: VPN over public internet between on-premises DC and AWS
- Direct Connect: direct private connection to AWS
- Transit Gateway: Connect thousands of VPC and on-premises networks together