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For the purposes of learning, I'm trying to setup a public/private VPC in AWS without using the wizard, and I'm trying to determine how AWS decides if a subnet is private/public.

For argument sake, let's say I have in my vpc 1 ig, 2 routing tables, 2 elbs, 4 instances, 4 subnets, and 2 availability zones.

[
    ig,
    routepub,
    routepriv (default),
    elbpub,
    elbpriv,
    { az1: [
      subnet1pub:  { range: "10.0.0.0/25",    instances: [ instance1pub  ] },
      subnet1priv: { range: "10.0.1.0/25",    instances: [ instance1priv ] }
    ] },
    { az2: [
      subnet2pub:  { range: "10.0.0.128/25",  instances: [ instance2pub  ] },
      subnet2priv: { range: "10.0.1.128/25",  instances: [ instance2priv ] }
    ] }
]

I have traffic coming from the greater internet to elbpub which then balances across my availability zones to instance1pub and instance2pub. I have some internal application (say a database or something such) on instance1priv and instance2priv which only instance1pub and instance2pub can access, and traffic to these is balanced via elbpriv.

When I try to set this up, I am told that I must attach my Internet-Facing ELB to a public subnet. So, how does AWS decide if a subnet is public or private?

And more specifically, I understand that private means it must go through a NAT and public means it goes through the IG, but how do I set up and/or link up my route tables/security groups to reflect this so that it lets me get public traffic into my public elb?

3

Only difference between the Private Subnets and Public Subnets is that, the latter one has connectivity to Internet Gateway established by the Subnet's Route table entry [ example : 10.0.10.0/24 ig-abcdef12 ].

So in your example you would put the ELB in front of the Instances which are in Public Subnet. For the record, you can also create an Internal Load Balancer [ for scenarios like Web Servers in public subnet talking to App Servers fronted by an ELB fully insider the VPC - Private Subnet ]

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