# Application Load Balancer (ALB)

* An **ALB functions at the application layer (the seventh layer of the OSI model)**.\\

* After the load balancer receives a request, it **evaluates the listener rules in priority order to determine which rule to apply, and then selects a target from the target group for the rule action**.

## **Host conditions**

* You can use host conditions to **define rules that route requests based on the host name in the host header (also known as host-based routing)**.\\

* This enables you to **support multiple subdomains and different top-level domains using a single load balancer**.

#### **Example hostnames**

* `example.com`
* `test.example.com`
* `hello.example.com`

## Path conditions

* You can use path conditions to **define rules that route requests based on the URL in the request (also known as path-based routing)**.\\

* The **path pattern is applied only to the path of the URL**, not to its query parameters.

#### **Example HTTP path patterns**

* `/img/*`
* `/img/*/pics`
