Who would use such a term…??
What purpose is this WAN Server, is it an FTP server, a video server etc etc…
So fine you are using WAN server but what is it serving… Dinner??
See even Sob agrees that you are being technically correct but not explaining it down to the masses so its palatable. Sigh, IT elitists…
Well i think @mkx answered your question…
Same technique…
From the context of discussion in this thread I’m assuming that OP wanted to implement a temporary solution where he moved some server from one IP address to another one and he wanted to make sure clients can connect server until all clients start to use new address this way or another.
You won’t believe, but there are (brain-dead?) clients with server configured as IP address not FQDN … so it might take days (or even weeks) to re-configure all the clients. And if one wants all clients (those using old address and those using new address) to communicate with very same server instance, then there are only two possibilites: hairpin-NAT or reverse proxy (third possibility, redirect, is only available for certain protocols and even then some brain-dead clients might not support it). Reverse proxy is not available on ROS, so the choice is obvious.
Even if clients are not completely brain-dead and do support DNS, it still might cause considerable service disturbance if one can not deal with DNS records properly (reduction of TTL in several steps, change of record, verification everything works correctly, …).
All good, just trying to pry some gems from you for the user article to help the overall understanding…
You never did answer my question of what type of WAN server it is,… serves coffee??
The reason for my sarcasm is that I yearn for consistency and this statement was very clear to me…
Quote: "… in the case of masquerading (redirecting) WANIPs, then the case becomes the WAN client talking to LAN server via some foreign (to both client and server) IP address (and that communication needs to pass router in both directions)… " unquote.
The genius who wrote that seemed to be talking about a WAN client and a LAN server !!!
Conclusion, when you wrote its a WAN server, you were drunk out of your tree and slurring your words together… ;-PP
I wrote:
… and in my case it’s WAN client talking to WAN server via some foreign (to both client and server) IP address (and that communication needs to pass router in both directions).
and that’s exactly what I meant, word by word.
And I don’t see what’s the signifficance of information what exactly that server is serving. Can be bull shit for what it’s worth.
If it was WAN client talking to LAN server, then simple DST NAT would do. But that’s not what OP was asking for. So go back to post #1 and try to understand the requirements. OP did excellent job describing needs in plain words (I wonder who has habit of requiring that from forum users with questions 
) and I provided explanation why hairpin NAT fits the task at hand.
Trust me, I did before I made the observation " I have internal server "
Doesnt sound smell or touch like a WAN server.
I’m not the IT pro so you have to be technically accurate whereas I can be forgiven for trespassing on nomenclature…
Its clear in my head now, no such thing as a WAN server, which was the confusion I was trying to overcome.
I give up, explaining this to you seems mission impossible.
No worries, I dont blame you, few have the energy to keep up. 
Plus I am thicker than a post, so very few make a dent.
@mkx: Watch the master.
@anav: It’s really simple. Imagine yourself as computer enthusiast who was given big server for free. Server, meaning the big heavy noisy professional computer found in datacenters. You start to play with it, install some OS, web server and stuff. It’s great fun. So you put it in your pantry, because you don’t have space for it elsewhere. And it’s fine, because who would mind a server in there. First you run your own hobby website on it (you have the usual dstnat from your router’s public address), then you let your friends run their own websites there too, because it’s big server and it’s nothing for it. Everything is just great.
Until one day your wife appears in front of you and she doesn’t look happy at all. She explains to you that she accidently kicked that damn thing for the last time, that she had enough of the noise it’s making, adds something about electricity bill being too high, and long story short, that thing will be gone tomorrow, or else. Your sense of self-preservation kicks in, so you quickly buy a VPS (virtual server), transfer everything to it and with heavy heart turn off the good old physical server.
But there’s problem. There was too many things there and you didn’t have complete control over everything. Your website is fine, because you already updated DNS for your domain, so it now points to public address of new server. But your friends, whose websites you also hosted, have their own domains and you don’t have access to their DNS. And you can’t reach all of them so quickly. One is on holiday out of civilization, another was just hit by bus yesterday, you name it. It means that their websites will be offline until they are able to update their DNS. Unless…
And that’s the solution described is this thread. Clear?
I fear you’ve list @anav here
But kudos, it’s a great story. Sounds like the first guy, out of civilitation, might be lost in Nova Scotia and the other one was actually hit by snow plough?
Great story very entertaining, luv the explanation of the WAN based servers, now why couldnt mkx have done that… 
'cause I didn’t want to drag your better half into this mess. 