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Last, but not least, is private site-to-site connectivity, which in Azure means ExpressRoute. This is called private because the network traffic occurs over your network provider and does not go across the public Internet as it does with both site-to-site and point-to-site connectivity. This capability ensures that applications with privacy requirements can be developed and run on Azure. Also, in most cases, using ExpressRoute gives you increased reliability and speed and lower latency.

With this option, the network connects on your end to one of two scenarios: hardware colocated at an Exchange provider (such as Equinix or Level 3) or an additional site on your MPLS VPN-based WAN through a network server provider. The connection from the Exchange provider or the network service provider connects directly to Azure. A single ExpressRoute circuit can connect to multiple virtual networks in the same Azure geography.

Inbound bandwidth to Azure is always free. ExpressRoute now provides either metered or unmetered billing. If you have workloads in which huge amounts of data are leaving the Azure datacenter, using ExpressRoute with unmetered billing can significantly lower the cost of that data transfer. Depending on the provider you select, the bandwidth can range from 50 Mbps to 10 Gpbs.

This is the best solution for delivering enterprise-grade solutions. It is a good fit for applications or workloads that are mission critical to your company. The consistent network performance provided by ExpressRoute also makes it a good solution if you have SLAs in place with groups internal or external to your organization.

Source of Information : Microsoft Azure Essentials Fundamentals of Azure Second Edition