Is it a good practice to have all the ip's in a different subnet for an AAG setup as mentioned in the link below? When I wanted to setup a 2 node AAG, the windows team in my office built the cluster and sent it over to me but apparently, all the ip's belong to the same subnet. The 2 Node setup is within the same Datacenter. We plan to expand this with a 3rd node in another site at SanJose later on
There are a few things going on in your question. To start I presume you are referring to an Always On Availability Group, just to be clear. You shared a link about multi-subnet failover.
Multi-subnet failovers are a feature, not a requirement. So the two nodes in your data center don't need to be on different subnets. Even when you go to San Jose later, it depends on if the subnet is "stretched" there whether you even need to worry about multi-subnet failover or not.
There are also various network connections involved in an Always On Availability Group.
At the very least there needs to be a public Network that all the nodes can see each other on and receive traffic from the outside on. You could setup with just this one network. You can be, as your link indicates , on separate subnets, but if you are on one? That makes life easier.
You can also add (and I say should) other networks if you don't then this one public network handles all of the functions:
Cluster Communications/Heartbeat - this is a network where the windows cluster nodes can send heartbeat traffic. These don't need to communicate on the public network and just need to be routable to one another.
SQL Server Availability Group Replication Network - The Availability Group replicas talk to each other over a network. You don't have to configure a separate network, but many environments prefer to segregate their traffic to a network that only has this replication traffic. These NICs would not need to speak to the public or rest of the netowrk, and just need to be routable to each other.
You didn't ask about those extra networks, and it seemed like you were just talking about the public network, but I wanted to throw a few thoughts out.