A site-to-site VPN uses a tunnel mode connection between VPN gateways to protect traffic between two or more sites or locations. Site-to-site connections are commonly referred to as LAN-to-LAN (L2L) connections.
Simple Site-to-Site Design Scenario
Above figure show an example of site to site VPN, We can deploy IPSEC tunnel between router else between concentrator. Its up to administrator who comes on these conclusion by router is lower end or higher end, who will be responsible for NAT, if pix will be responsible for NAT in this feature concentrator must support the feature IPSEC NAT- Traversal feature (IPSEC NAT-T).
Hub-and-Spoke Networks and Site-to-Site VPN
Above figure is an example of HUB-SPOKE site to site VPN.
Example of Corporate Extranet and Site-to-Site VPN
Site-to-site VPN deployments are also popular in corporate extranets. When an organization requires dedicated site-to-site connectivity to a peer organization or subsidiary, often, a dedicated, high-speed WAN circuit is provisioned