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AWS EC2 Networking Basics

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Elastic IPs


Important to remember!
When you stop and then start an EC2 instance, it can change its public IP!


So if you need a fixed public IP for an instance, you must use an Elastic IP – or else register your own public IP and domain name


You own the Elastic IP as long as you don’t delete it, you can attach it to one instance.


but you can remap the ip address to another instance in your account in the event of an instance or software failure.


But – you can only have max 5 Elastic IPs in your account.


Best practice: DON’T use Elastic IP but instead use your own registered public ip with a registered domain name. You can do this via AWS Route 53.



Even better, you can use a load balancer to connect to the private ip addresses of your instances.


Always remember – your EC2-allocated public ip will change if you shutdown and restart the instance!


The *private* iP however always remains the same.


You allocate an Elastic IP to an EC2 instance from your EC2 Dashboard.




EC2 Placement Groups (PGs)



these give you control over where your EC2 instances are placed.


This gives you a way to specify where you want your EC2s to be physically located.


Placement groups can be cluster, spread or partition-based





all in same hw rack
in same AZ


pro: network speeds high between instances
con: high risk of total failure if the rack fails – because all EC2 instances will fail together


use cases: best for big data that must be quickly processed




this minimizes risk compared to cluster PG


instances across different AZs
and across different hw racks


pro: less risk of failure


con: you are allowed max 7 instances per AZ per PG permitted


use cases: best for high availability, critical applications that must be isolated from failure from each other





each partition = one rack


you can have multiple partitions – 7 maximum in each AZ in same region


100s of EC2s per partition possible


– failure can affect other EC2s on same rack, but not other partitions


can use metadata service to get info about other EC2s on the partition


use cases include apache-kafka, hdfs, cassandra




ENI Elastic Network Interfaces


This is the AWS virtual network interface


it can have


one private primary IPv4 address


one or more secondary IPv4 addresses


one Elastic IP per private IPv4


one public IPv4


one or more security groups


a MAC address


you can move ENIs to other instances of EC2 on failover!


but note – ENIs are bound to a specific AZ







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