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DNS geolocation for Office 365, connecting you to your nearest Datacenter for the fastest connectivity

IMPORTANT UPDATE - This method of connectivity for Exchange Online is no longer in effect. Microsoft uses an Anycast DNS model for Exchange Online connectivity which I'll describe in another post. This GeoDNS model is still relevant as it is available as a backup should it be required so I've left the post for reference


One of the main things we need to get right to ensure the most efficient and speedy connectivity to O365 is where in the world your DNS call is being completed. You'd think this wouldn't matter, you do a DNS lookup for your O365 tenant, get the address then connect right? Well, normally yes, but with O365, especially with Outlook, we do some pretty clever stuff to utilise our worldwide array of datacenters to ensure you get connected to your data as efficiently as possible.

Your Outlook connection will do a DNS lookup and we use the location of that lookup to connect you to your nearest Microsoft Datacenter. With Outlook we'll connect to a CAS server there and use our fast Datacenter to datacenter backbone network to connect you to the datacenter where your exchange servers (and data) are located. This generally works much quicker than a direct connection to the datacenter where your tenant is located due to the speed of the interconnecting networks we have. outlines this in more detail but a diagram nicked from this post shows how this works for Outlook/Exchange connectivity when the Exchange mailbox is located in a NA datacenter but the user is physically located in EMEA. Therefore the DNS lookup is performed in EMEA, we connect to the nearest EMEA datacenter, which then routes the connection through to your mailbox over our backbone network, all in the background and your Outlook client knows nothing about this magic going on behind the scenes.


If your environment is making its DNS calls in a location on a different continent to where the user is physically located then you are going to get really bad performance with O365. Take an example where the user and Mailbox is located in EMEA. Your company uses DNS servers located in the USA for all calls, or the user is incorrectly set to use a proxy server in the USA, thus we're given the IP address of a USA based datacenter as that's where we think your user is located. The client will then connect to the USA based datacenter which will route the traffic to the EMEA datacenter which will then send the response back to the USA based datacenter which will then respond to the client back in EMEA. So with this scenario we've got several unnecessary trips across the pond with our data.

It is therefore vitally important to get the DNS lookup right for when you move to Outlook on Office 365.

So how do you check this? Well it could be a bit tricky as although we release a list of IP addresses used for O365, we don't tell you which ones map to where, for many reasons including the fact they change regularly. Thankfully one of my Microsoft colleagues has shown me an easy way to check you're connecting to a local datacenter.

All you need to do is open a command prompt on the client and ping and the response will tell you where the datacenter is you'll connect to. So sat here in the UK at home, I get EMEAWEST


If I connect to our Singapore VPN endpoint and turn off split tunnelling and force the DNS call down the VPN link (our Internal IT do a great job of making these things configurable for us techies) then I get directed to apacsouth.

And if I connect via VPN to the mothership in Seattle, my DNS call is completed there and thus I get directed to namnorthwest.

So it's a quick and easy check, just make sure the datacenter returned is in the same region as you're physically located in.

SharePoint is currently directed to the datacenter where your tenant is located so it doesn't matter so much where the call is made for this (although it should still preferably be local to the user for the portal connection). Lync is slightly different and is outlined in this article in more detail.

It's also worth ensuring all your clients are using a proxy in the same region as where they are located, as if not, they could hit the problem outlined above and thus be getting unnecessarily poor O365 performance.