We have four hotels in different locations, and all hotels have their own server. The main server is in another hotel. Now I want to access applications from the main server at another hotel.
How can I connect them?
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Virtual Private Network (VPN) over the Internet with IPSec protocols for authentication and encryption. Using a Firewall w/VPN is typical here. A hub-and-spoke model is generally deployed. With hub-and-spoke, all VPN tunnels from each hotel (spoke) connects to the main hotel (hub), and spokes cannot talk directly to one another.
VPN provided by your Service Provider to tie all your sites together on private networks. This can be done by logically separating your DIA (Direct Internet Access) traffic from your VPN. Use a provider-managed router that you'll need to "rent" unless you're at an advanced level to tackle this yourself.
App exposure over Internet
Expose main server applications over Internet via your main site firewall or load-balancer. Require SSL on top of app and require application login authentication. Could restrict access on firewall by only allowing the public source IP blocks of the hotels.
PtP (Point-to-Point) links
These could be various technologies using dedicated circuits to the main site (hub) or Metro Ethernet.
IP over Avian Carriers (IPoAC)
IPoAC has been successfully implemented, but for only nine packets of data, with a packet loss ratio of 55% (due to user error), and a response time ranging from 3000 seconds (~54 minutes) to over 6000 seconds (~1.77 hours).
See IPoAC on Wikipedia
I suppose you don't want to put public addresses on your server, so you need to create a network connecting your different locations. Either purchase a VPN from your ISP or build your own IPsec VPN.
To get a VPN from your ISP, ask for a "IP VPN" or "L3VPN" according to RFC2547/RFC4364. Feel free to throw in "MPLS" in the same sentence. They will often be able to provide you your VPN connectivity over a VLAN on the same physical circuits over which you purchase your Internet connectivity.
IPSec means you need support in your routers and you need to manually configure those yourself (or bring in a consultant). Beware of MTU issues. Google is your friend :)
Depending on your ISP technology, you have several options. An IPSec or SSL VPN configured by you is the cheapest ISP-independant way to get you going. You would need an edge device to NAT, encrypt and authenticate your packets. The downside for this solution is that you need to have the expertise (or get someone who does) and depending on your current setup, get some new equipment. You wouldn't also have guaranteed bandwidth.
If you call your ISP and ask for a "site connection product" they might sell you L2VPN or L3VPN, either way the configuration is going to be pretty straight-forward (just adding a new route or just connecting the new equipment to a switch) and your ISP might provide for this extra equipment, besides, if you have a 10mbps contract, you're going to have 10mbps bandwidth, no matter the congestion on the network. The ISP will charge you more than your IPSec / SSL VPN costs for this setup.