Now that you know about DC and DR locations and the data is being copied from DC to DR in near Realtime at high speed, this is only possible if we have connectivity between them so how are they connected?

It’s simple you engage an ISP "internet service provider" and ask him for connectivity between the two sites (DC and DR). The ISP pulls out a cable, attaches one end of the cable to one site (maybe Mumbai) and the other side of the cable to the other Site (Maybe Chennai), see its simple to understand, isn’t it?, You would think either I am a fool or trying to fool you how can we use a single cable and attach one end to one geographic location and another end to another location, yes I know that’s not possible but when I say DC to DR connectivity then automatically you should be imagining that there is a longggg cable which is connecting the 2 sites.

All the ISPs ride on the underground Fiber cables being laid, it can be their own cables or they would be taking it on lease from another ISP; in any case you as an enterprise get to purchase the lines on lease from the ISP agreeing on a particular bandwidth and use these lines to transfer data, this type of connectivity between Datacenters or from Datacenter to Offices or from Office to Office is called WAN (Wide Area Network) connectivity

You know what; again, the specific usage of a particular term is important in this regard, WAN word would be mostly used between DC to DR connectivity and the DC to Offices would be spoken as LAN although in reality it’s a WAN connection, so it’s very important that you understand the specific use of the IT vocabulary because many times the actual meaning of the words are not followed

Note the DC and DR WAN link is mostly used for replicating (copying) data from DC to DR so that the DR would always remain in sync but there is definitely some lag between DC and DR status of data as it takes time to copy data over a WAN link between 2 locations which are geographically apart

I guess you now have a good grasp on the meaning of WAN, let us at the same time understand what LAN (Local Area Network) is, LAN is any network which is internal or local, best example to understand would be your home Wifi network, all the users, be it mobile or Laptop in your home using the same Wifi are actually on the same LAN network, a bit of technical meaning would be that all the users in your home can reach each other's devices directly without passing via a router (gateway), what I mean is that if you plug out the internet cable which is coming from your ISP still all members connected to the WIFI can reach each other, you are actually getting on internet only once you cross the gateway, similarly when you are in your office network, all employees within the building are connected to a single LAN and if they want to reach a particular application in DC or the internet they have to cross their gateways (ideally it’s the default gateway IP configured in your laptop) so every office would have some kind of router which connects to the WAN to connect to the DC or the other offices

So, in a Nutshell a LAN is any network which is local to all the devices, in fact all your devices / servers present in a single datacenter are in the same LAN

The most happening technology for WAN in today’s world is MPLS (Multi-Protocol Label Switching), I won’t dive deep into this tech here but the key points about this tech are that it’s a Shared cloud which multiple customers use and for packet transfer it uses an Overlay of "Labels" on top of the IP addresses and routing table

Note: In the diagrams, I am not showing the Internet links as I want to keep the diagrams less complex, the Internet link would also come under the WAN link as you may have already understood it